Basic Math and Pre-Algebra Workbook for Dummies
Author: Mark Zegarelli
Date: 2014-02-28T05:00:00+00:00
Keywords: Mathematics; Algebra; General
Description: Basic Math and Pre-Algebra Workbook For Dummies, 2nd Edition helps take the guesswork out of solving math equations and will have you unraveling the mystery of FOIL in no time. Whether you need to brush up on the basics of addition, subtraction, multiplication, and division or you’re ready to tackle algebraic expressions and equations, this handy workbook will demystify math so you can get back to having fun in math class. - Properly use negative numbers, units, inequalities, exponents, square roots, and absolute value - Round numbers and estimate answers - Solve problems with fractions, decim
Publisher: For Dummies

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Basic Math &

Pre-Algebra

Workbook

FOR

‰

DUMmIES
by Mark Zegarelli

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Basic Math & Pre-Algebra Workbook For Dummies®

Published by

Wiley Publishing, Inc.

111 River St.

Hoboken, NJ 07030-5774
www.wiley.com

Copyright © 2008 by Wiley Publishing, Inc., Indianapolis, Indiana

Published by Wiley Publishing, Inc., Indianapolis, Indiana

Published simultaneously in Canada

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Library of Congress Control Number: 2008932076

ISBN: 978-0-470-28817-7

Manufactured in the United States of America

10 9 8 7 6 5 4 3 2 1

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Contents at a Glance

Introduction .............................................................................1

Part I: Back to Basics with Basic Math......................................5

Chapter 1: We’ve Got Your Numbers...................................................................................................7

Chapter 2: Smooth Operators: Working with the Big Four Operations ........................................23

Chapter 3: Getting Down with Negative Numbers ...........................................................................37

Chapter 4: It’s Just an Expression......................................................................................................49

Chapter 5: Dividing Attention: Divisibility, Factors, and Multiples ...............................................69

Part II: Slicing Things Up: Fractions, Decimals, and Percents ....89

Chapter 6: Fractions Are a Piece of Cake..........................................................................................91

Chapter 7: Fractions and the Big Four ............................................................................................107

Chapter 8: Getting to the Point with Decimals...............................................................................141

Chapter 9: Playing the Percentages.................................................................................................163

Part III: A Giant Step Forward: Intermediate Topics ................175

Chapter 10: Seeking a Higher Power through Scientific Notation ...............................................177

Chapter 11: Weighty Questions on Weights and Measures ..........................................................187

Chapter 12: Shaping Up with Geometry..........................................................................................201

Chapter 13: Getting Graphic: Cartesian Graphs.............................................................................221

Part IV: The X Factor: Introducing Algebra .............................233

Chapter 14: Expressing Yourself with Algebraic Expressions......................................................235

Chapter 15: Finding the Right Balance: Solving Algebraic Equations .........................................255

Part V: The Part of Tens ........................................................271

Chapter 16: Ten Alternative Numeral and Number Systems........................................................273

Chapter 17: Ten Curious Types of Numbers ..................................................................................281

Index ...................................................................................287

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Table of Contents

Introduction..............................................................................1

About This Book ..................................................................................................................1

Conventions Used in This Book.........................................................................................2

Foolish Assumptions...........................................................................................................2

How This Book Is Organized ..............................................................................................2

Part I: Back to Basics with Basic Math....................................................................2

Part II: Slicing Things Up: Fractions, Decimals, and Percents..............................3

Part III: A Giant Step Forward: Intermediate Topics..............................................3

Part IV: The X Factor: Introducing Algebra ............................................................3

Part V: The Part of Tens ............................................................................................3

Icons Used in This Book .....................................................................................................4

Where to Go from Here .......................................................................................................4

Part I: Back to Basics with Basic Math ......................................5

Chapter 1: We’ve Got Your Numbers .............................................................................7

Getting in Place with Numbers and Digits........................................................................7

Rollover: Rounding Numbers Up and Down ..................................................................10

Using the Number Line with the Big Four ......................................................................12

The Column Lineup: Adding and Subtracting................................................................14

Multiplying Multiple Digits ...............................................................................................15

Cycling through Long Division.........................................................................................17

Solutions to We’ve Got Your Numbers............................................................................19

Chapter 2: Smooth Operators: Working with the Big Four Operations.................23

Switching Things Up with Inverse Operations and the Commutative Property .......23

Getting with the In-Group: Parentheses and the Associative Property......................27

Becoming Unbalanced: Inequalities ................................................................................30

Special Times: Powers and Square Roots.......................................................................32

Answers to Problems in Smooth Operators...................................................................34

Chapter 3: Getting Down with Negative Numbers ...................................................37

Understanding Where Negative Numbers Come From.................................................37

Sign-Switching: Understanding Negation and Absolute Value .....................................39

Adding with Negative Numbers .......................................................................................40

Subtracting with Negative Numbers ...............................................................................42

Comparing Signs of the Times (And Division) of Negative Numbers .........................43

Answers to Problems in Getting Down with Negative Numbers .................................45

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Basic Math & Pre-Algebra Workbook For Dummies

Chapter 4: It’s Just an Expression................................................................................49

Evaluating Expressions with Addition and Subtraction ...............................................49

Evaluating Expressions with Multiplication and Division ............................................51

Making Sense of Mixed-Operator Expressions ..............................................................52

Handling Powers Responsibly .........................................................................................53

Prioritizing Parentheses ...................................................................................................54

Pulling Apart Parentheses and Powers...........................................................................56

Figuring Out Nested Parentheses ....................................................................................57

Bringing It All Together: The Order of Operations........................................................59

Solutions to It’s Just an Expression.................................................................................60

Chapter 5: Dividing Attention: Divisibility, Factors, and Multiples .......................69

Checking for Leftovers: Divisibility Tests.......................................................................69

Understanding Factors and Multiples.............................................................................72

One Number, Indivisible: Identifying Prime (And Composite) Numbers ...................73

Generating a Number’s Factors .......................................................................................75

Decomposing a Number into Its Prime Factors.............................................................77

Finding the Greatest Common Factor (GCF)..................................................................79

Generating the Multiples of a Number............................................................................81

Finding the Least Common Multiple (LCM) ...................................................................82

Solutions to Divisibility, Factors, and Multiples ............................................................84

Part II: Slicing Things Up: Fractions, Decimals, and Percents.....89

Chapter 6: Fractions Are a Piece of Cake ..................................................................91

Getting Down the Basic Fraction Stuff ............................................................................91

In Mixed Company: Converting between Mixed Numbers and

Improper Fractions ........................................................................................................94

Increasing and Reducing the Terms of Fractions ..........................................................97

Comparing Fractions with Cross-Multiplication..........................................................100

Solutions to Fractions Are a Piece of Cake...................................................................102

Chapter 7: Fractions and the Big Four.......................................................................107

Multiplying Fractions: A Straight Shot ..........................................................................107

Flipping for Fraction Division.........................................................................................109

Reaching the Common Denominator: Adding Fractions ............................................111

The Other Common Denominator: Subtracting Fractions .........................................115

Multiplying and Dividing Mixed Numbers....................................................................118

Carried Away: Adding Mixed Numbers.........................................................................120

Borrowing from the Whole: Subtracting Mixed Numbers ..........................................123

Solutions to Fractions and the Big Four .......................................................................127

Chapter 8: Getting to the Point with Decimals........................................................141

Getting in Place: Basic Decimal Stuff.............................................................................141

Knowing Simple Decimal-Fraction Conversions ..........................................................144

A New Lineup: Adding and Subtracting Decimals .......................................................146

Counting Decimal Places: Multiplying Decimals .........................................................148

Decimal Points on the Move: Dividing Decimals .........................................................149

Changing Decimals to Fractions ....................................................................................151

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Table of Contents
xi

Changing Fractions to Decimals ....................................................................................153

Solutions to Getting to the Point with Decimals..........................................................155

Chapter 9: Playing the Percentages..........................................................................163

Converting Percents to Decimals ..................................................................................163

Changing Decimals to Percents .....................................................................................165

Switching from Percents to Fractions...........................................................................166

Converting Fractions to Percents..................................................................................167

Mixing Things Up with the Percent Circle....................................................................169

Solutions to Playing the Percentages............................................................................172

Part III: A Giant Step Forward: Intermediate Topics.................175

Chapter 10: Seeking a Higher Power through Scientific Notation .....................177

On the Count of Zero: Understanding Powers of Ten .................................................177

Exponential Arithmetic: Multiplying and Dividing Powers of Ten ............................180

Representing Numbers in Scientific Notation..............................................................181

Multiplying and Dividing with Scientific Notation.......................................................182

Answers to Problems in Seeking a Higher Power through Scientific Notation .......184

Chapter 11: Weighty Questions on Weights and Measures .................................187

The Basics of the English System ..................................................................................187

Going International with the Metric System ................................................................190

Converting between English and Metric Units ............................................................193

Answers to Problems in Weighty Questions on Weights and Measures ..................195

Chapter 12: Shaping Up with Geometry....................................................................201

Getting in Shape: Polygon (And Nonpolygon) Basics .................................................201

Making a Triple Play with Triangles ..............................................................................202

Taking a Fourth Side with Quadrilaterals .....................................................................205

Getting Around with Circle Measurements ..................................................................210

Building Solid Measurement Skills ................................................................................212

Answers to Problems in Shaping Up with Geometry ..................................................217

Chapter 13: Getting Graphic: Cartesian Graphs ......................................................221

Getting the Point of the Cartesian Graph .....................................................................221

Drawing the Line on the Cartesian Graph ....................................................................225

Answers to Problems in Getting Graphic: Cartesian Graphs.....................................228

Part IV: The X Factor: Introducing Algebra..............................233

Chapter 14: Expressing Yourself with Algebraic Expressions.............................235

Plug It In: Evaluating Algebraic Expressions ................................................................236

Knowing the Terms of Separation .................................................................................237

Adding and Subtracting Similar Terms .........................................................................239

Multiplying and Dividing Terms.....................................................................................240

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xii Basic Math & Pre-Algebra Workbook For Dummies

Simplifying Expressions by Combining Similar Terms ...............................................242

Simplifying Expressions with Parentheses...................................................................244

FOILing: Dealing with Two Sets of Parentheses...........................................................245

Answers to Problems in Expressing Yourself with Algebraic Expressions ..............248

Chapter 15: Finding the Right Balance: Solving Algebraic Equations................255

Solving Simple Algebraic Equations..............................................................................255

Equality for All: Using the Balance Scale to Isolate x..................................................258

Switching Sides: Rearranging Equations to Isolate x ..................................................259

Barring Fractions: Cross-Multiplying to Simplify Equations ......................................261

Answers to Problems in Finding the Right Balance: Solving

Algebraic Equations.....................................................................................................264

Part V: The Part of Tens.........................................................271

Chapter 16: Ten Alternative Numeral and Number Systems ................................273

Tally Marks .......................................................................................................................273

Bundled Tally Marks........................................................................................................274

Egyptian Numerals ..........................................................................................................274

Babylonian Numerals ......................................................................................................275

Ancient Greek Numerals .................................................................................................275

Roman Numerals .............................................................................................................276

Mayan Numerals ..............................................................................................................276

Base 2, or Binary Numbers.............................................................................................277

Base 16, or Hexadecimal Numbers................................................................................278

Prime-Based Numbers ....................................................................................................279

Chapter 17: Ten Curious Types of Numbers..............................................................281

Square Numbers ..............................................................................................................281

Triangular Numbers ........................................................................................................282

Cubic Numbers ................................................................................................................283

Factorial Numbers ...........................................................................................................283

Powers of Two..................................................................................................................284

Perfect Numbers ..............................................................................................................284

Amicable Numbers ..........................................................................................................285

Prime Numbers ................................................................................................................285

Mersenne Primes .............................................................................................................286

Fermat Primes ..................................................................................................................286

Index....................................................................................287

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Introduction

When you approach math right, it’s almost always easier than you think. And a lot of the stuff that hung you up when you first saw it probably isn’t all that scary after all.

Lots of students feel they got lost somewhere along the way on the road between learning to count to ten and their first day in an algebra class — and this may be true whether you’re 14 or 104. If this is you, don’t worry. You’re not alone, and help is right here!

Basic Math & Pre-Algebra Workbook For Dummies can give you the confidence and math skills you need to succeed in any math course you encounter on the way to algebra. One of the easiest ways to build confidence is to get experience working problems, allowing you to build those skills quickly. Everything in this book is designed to help clear the path on your math journey. Every section of every chapter contains a clear explanation of what you need to know, with plenty of practice problems and step-by-step solutions to every problem. Just grab a pencil, open this book to any page, and begin strengthening your math muscles!

About This Book

This book is for anyone who wants to improve his or her math skills. You may already be enrolled in a math class or preparing to register for one or simply studying on your own. In any case, practice makes perfect, and in this book you get plenty of practice solving a wide variety of math problems.

Each chapter covers a different topic in math: negative numbers, fractions, decimals, geometry, graphing, basic algebra — it’s all here. In every section within a chapter, you find problems that allow you to practice a different skill. Each section features the following: ߜ A brief introduction to that section’s topic

ߜ An explanation of how to solve the problems in that section

ߜ Sample questions with answers that show you all the steps to solving the problem ߜ Practice problems with space to work out your answer

Go ahead and write in this book — that’s what it’s for! When you’ve completed a problem or group of problems, flip to the end of the chapter. You’ll find the correct answer followed by a detailed, step-by-step explanation of how to get there.

Although you can certainly work all the exercises in this book from beginning to end, you don’t have to. Feel free to jump directly to whatever chapter has the type of problems you want to practice. When you’ve worked through enough problems in a section to your satisfac-tion, feel free to jump to a different section. If you find the problems in a section too difficult, flip back to an earlier section or chapter to practice the skills you need — just follow the cross-references.

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2 Basic Math & Pre-Algebra Workbook For Dummies

Conventions Used in This Book

Throughout the book, I stick to the following conventions:

ߜ Italicized text highlights new words and defined terms.

ߜ Boldfaced text indicates keywords in bulleted lists and the action part of numbered steps.

ߜ As is common in algebra, I use the · symbol rather than × to represent multiplication.

ߜ Variables, such as x and y, are in italics.

Foolish Assumptions

You probably realize that the best way to figure out math is by doing it. You only want enough explanation to get down to business so you can put your math skills to work right away. If so, you’ve come to the right place. If you’re looking for a more in-depth discussion, including tips on how all these math concepts fit into word problems, you may want to pick up the companion book, Basic Math & Pre-Algebra For Dummies.

I’m willing to bet my last dollar on earth that you’re ready for this book. I assume only that you have some familiarity with the basics of the number system and the Big Four operations (adding, subtracting, multiplying, and dividing). To make sure that you’re ready, take a look at these four arithmetic problems and see whether you can answer them:

3 + 4 = ________________

10 – 8 = ________________

5 · 5 = ________________

20 ÷ 2 = ________________

If you can do these problems, you’re good to go!

How This Book Is Organized

The book is divided into five parts. Each part is broken into chapters discussing a key topic in math. Here’s an overview of what’s covered.

Part I: Back to Basics with Basic Math

Part I starts at the very beginning of math, reviewing topics you may be familiar with and introducing important math ideas that you just can’t live without. Chapter 1 focuses on the counting numbers, and Chapter 2 provides you with a deeper look at the Big Four operations.

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Introduction

3

In Chapter 3, the focus is on all things negative — that is, negative numbers. In Chapter 4, you discover how to work with and evaluate expressions, which are strings of symbols that you can place on one side of an equal sign in an equation. Chapter 5 wraps up this part with a discussion of divisibility.

Part II: Slicing Things Up: Fractions,

Decimals, and Percents

In Part II, you discover three ways to talk about parts of the whole: fractions, decimals, and percents. In Chapter 6, I cover the basics of working with fractions, and Chapter 7 builds on those skills as you apply the Big Four operations (adding, subtracting, multiplying, and dividing) to fractions. Chapter 8 focuses on decimals, and Chapter 9 covers percents.

Part III: A Giant Step Forward:

Intermediate Topics

Here, you get to take a giant step forward in your problem-solving skills. The cool thing about this part is that it gives you some simple techniques for dealing with problems that have real-world applications, whether in science, in business, or at the hardware store. Chapter 10
covers scientific notation, which allows you to express very large and very small numbers efficiently. In Chapter 11, the topic is weights and measures, specifically the English system and the metric system. Chapter 12 focuses on geometry. Finally, in Chapter 13, I discuss the oh-so-practical skill of reading and understanding graphs.

Part IV: The X Factor: Introducing Algebra

This is what the pre-algebra buildup is all about. In Chapter 14, you make the leap from arithmetic to algebra. I show you how an algebraic expression is similar to an arithmetic expression, except that it has at least one variable such as x or y. You discover how to evaluate and simplify algebraic expressions, and I show you how to apply the Big Four operations (adding, subtracting, multiplying, and dividing) to algebraic expressions. In Chapter 15, all your skills from Chapter 14 come into play, and I show you how to solve algebraic equations, which is an amazingly powerful tool.

Part V: The Part of Tens

Every For Dummies book, even a math workbook like this one, ends with some fun top-ten lists.

In Chapter 16, I show you ten systems of numerals that differ from the standard Hindu-Arabic (or decimal) system. Chapter 17 covers a variety of curious sets of numbers. Trust me — it’s all very interesting.

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4 Basic Math & Pre-Algebra Workbook For Dummies

Icons Used in This Book

Throughout this book, I’ve highlighted some of the most important information with a variety of icons. Here’s what they all mean:

This icon points out some of the most important pieces of information. Pay special attention to these details — you need to know them!

Tips show you a quick and easy way to do a problem. Try these tricks as you’re solving the problems in that section.

Warnings are math booby traps that unwary students often fall into. Reading these bits carefully can help you avoid unnecessary heartache.

This icon highlights the example problems that show you techniques before you dive into the exercises.

Where to Go from Here

You can turn to virtually any page in this book and begin improving your math skills. Here are a few chapters where I discuss topics that tend to hang up math students: ߜ Chapter 3: Negative numbers

ߜ Chapter 4: Order of operations

ߜ Chapter 5: Factors and multiples

ߜ Chapter 6: Fractions

A lot of what follows later in the book builds on these important early topics, so check them out. When you feel comfortable doing these types of problems, you have a real advantage in any math class.

Of course, if you already have a good handle on these topics, you can go anywhere you want (though you may still want to skim these chapters for some tips and tricks). My only advice is that you do the problems before reading the answer key!

And by all means, while you’re at it, pick up Basic Math & Pre-Algebra For Dummies, which contains more-detailed explanations and a few extra topics not covered in this workbook.

Used in conjunction, these two books can provide a powerful one-two punch to take just about any math problem to the mat.

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Part I

Back to Basics
with Basic Math

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In this part . . .

Here, you rediscover and practice some basics that
can help improve your math skills as you move for-
ward. You review important concepts such as place value,
the Big Four operations (adding, subtracting, multiplying,
and dividing), positive and negative numbers, the order of
operations, and factors and multiples.

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Chapter 1

We’ve Got Your Numbers

In This Chapter

ᮣ Understanding how place value turns digits into numbers

ᮣ Rounding numbers to the nearest ten, hundred, or thousand

ᮣ Calculating with the Big Four operations (adding, subtracting, multiplying, and dividing)

ᮣ Getting comfortable with long division

In this chapter, I give you a review of basic math, and I do mean basic. I bet you know a lot of this stuff already. So consider this a trip down memory lane, a mini-vacation from whatever math you may be working on right now. With a really strong foundation in these areas, you’ll find the chapters that follow a lot easier.

First, I discuss how the number system you’re familiar with — called the Hindu-Arabic number system (or decimal numbers) — uses digits and place value to express numbers.

Next, I show you how to round numbers to the nearest ten, hundred, or thousand.

After that, I discuss the Big Four operations: adding, subtracting, multiplying, and dividing.

You see how to use the number line to make sense of all four operations. Then I give you practice doing calculations with larger numbers. To finish up, I make sure you know how to do long division both with and without a remainder.

Algebra often uses the dot (·) in place of the times sign (×) to indicate multiplication, so that’s what I use in this book.

Getting in Place with Numbers and Digits

The number system used most commonly throughout the world is the Hindu-Arabic number system. This system contains ten digits (also called numerals), which are symbols like the letters A through Z. I’m sure you’re quite familiar with them: 1

2

3

4

5

6

7

8

9

0

Like letters of the alphabet, individual digits aren’t very useful. When used in combination, however, these ten symbols can build numbers as large as you like using place value. Place value assigns each digit a greater or lesser value depending upon where it appears in a number. Each place in a number is ten times greater than the place to its immediate right.

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8 Part I: Back to Basics with Basic Math

Although the digit 0 adds no value to a number, it can act as a placeholder. When a 0 appears to the right of at least one non-zero digit, it’s a placeholder. Placeholders are important for giving digits their proper place value. In contrast, when a 0 isn’t to the right of any nonzero digit, it’s a leading zero. Leading zeros are unnecessary and can be removed from a number.

Q. In the number 284, identify the ones digit, the tens digit, and the hundreds digit.

A. The ones digit is 4, the tens digit is 8, and the hundreds digit is 2.

Q. Place the number 5,672 in a table that shows the value of each digit. Then use this table and an addition problem to show how this number breaks down digit by digit.

A.

Millions

Hundred Thousands

Ten Thousands

Thousands

Hundreds

Tens

Ones

5

6

7

2

The numeral 5 is in the thousands place, 6 is in the hundreds place, 7 is in the tens place, and 2 is in the ones place, so here’s how the number breaks down:

5,000 + 600 + 70 + 2 = 5,672

Q. Place the number 040,120 in a table that shows the value of each digit. Then use this table to show how this number breaks down digit by digit. Which 0s are placeholders, and which are leading zeros?

A.

Millions

Hundred Thousands

Ten Thousands

Thousands

Hundreds

Tens

Ones

0

4

0

1

2

0

The first 0 is in the hundred-thousands place, 4 is in the ten-thousands place, the next 0 is in the thousands place, 1 is in the hundreds place, 2 is in the tens place, and the last 0 is in the ones place, so

0 + 40,000 + 0 + 100 + 20 + 0 = 40,120

The first 0 is a leading zero, and the remaining 0s are placeholders.

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Chapter 1: We’ve Got Your Numbers

9

1. In the number 7,359, identify the following digits:

a. The ones digit
b. The tens digit
c. The hundreds digit
d. The thousands digit

Solve It

2. Place the number 2,136 in a table that shows the value of each digit. Then use this table to show how this number breaks down digit by digit.

Millions

Hundred Thousands

Ten Thousands

Thousands

Hundreds

Tens

Ones

Solve It

3. Place the number 03,809 in a table that shows the value of each digit. Then use this table to show how this number breaks down digit by digit. Which 0 is a placeholder and which is a leading zero?

Millions

Hundred Thousands

Ten Thousands

Thousands

Hundreds

Tens

Ones

Solve It

4. Place the number 0,450,900 in a table that shows the value of each digit. Then use this table to show how this number breaks down digit by digit. Which 0s are placeholders and which are leading zeros?

Millions

Hundred Thousands

Ten Thousands

Thousands

Hundreds

Tens

Ones

Solve It

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10 Part I: Back to Basics with Basic Math

Rollover: Rounding Numbers Up and Down

Rounding numbers makes long numbers easier to work with. To round a two-digit number to the nearest ten, simply bring it up or down to the nearest number that ends in 0: ߜ When a number ends in 1, 2, 3, or 4, bring it down; in other words, keep the tens digit the same and turn the ones digit into a 0.

ߜ When a number ends in 5, 6, 7, 8, or 9, bring it up; add 1 to the tens digit and turn the ones digit into a 0.

To round a number with more than two digits to the nearest ten, use the same method, focusing only on the ones and tens digits.

After you understand how to round a number to the nearest ten, rounding a number to the nearest hundred, thousand, or beyond is easy. Focus only on two digits: The digit in the place you’re rounding to and the digit to its immediate right, which tells you whether to round up or down. All the digits to the right of the number you’re rounding to change to 0s.

Occasionally when you’re rounding a number up, a small change to the ones and tens digits affects the other digits. This is a lot like when the odometer in your car rolls a bunch of 9s over to 0s, such as when you go from 11,999 miles to 12,000 miles.

Q. Round the numbers 31, 58, and 95 to the

Q. Round the numbers 742, 3,820, and
nearest ten.

61,225 to the nearest ten.

A. 30, 60, and 100.

A. 740, 3,820, and 61,230.

The number 31 ends in 1, so round it

The number 742 ends in 2, so round it
down:

down:

31 → 30

742 → 740

The number 58 ends in 8, so round it up:

The number 3,820 already ends in 0, so
no rounding is needed:

58 → 60

3,820 → 3,820

The number 95 ends in 5, so round it up:

The number 61,225 ends in 5, so round

95 → 100
it up:

61,225 → 61,230

05_288177-ch01.qxp 7/28/08 12:57 PM Page 11

Chapter 1: We’ve Got Your Numbers

11

5. Round these two-digit numbers to the

6. Round these numbers to the nearest ten:

nearest ten:

a. 164
a. 29
b. 765
b. 43
c. 1,989
c. 75
d. 9,999,995
d. 95

Solve It

Solve It

7. Round these numbers to the nearest

8. Round these numbers to the nearest
hundred:

thousand:

a. 439
a. 5,280
b. 562
b. 77,777
c. 2,950
c. 1,234,567
d. 109,974
d. 1,899,999

Solve It

Solve It

05_288177-ch01.qxp 7/28/08 12:57 PM Page 12

12 Part I: Back to Basics with Basic Math

Using the Number Line with the Big Four

The number line is just a line with numbers marked off at regular intervals. You probably saw your first number line when you were first figuring out how to count to ten. In this section, I show you how to use this trusty tool to perform the Big Four operations (adding, subtracting, multiplying, and dividing) on relatively small numbers.

The number line can be a useful tool for adding and subtracting small numbers: ߜ When you add, move up the number line, to the right.

ߜ When you subtract, move down the number line, to the left.

To multiply on the number line, start at 0 and count by the first number in the problem as many times as indicated by the second number.

To divide on the number line, first block off a segment of the number line from 0 to the first number in the problem. Then divide this segment evenly into the number of pieces indicated by the second number. The length of each piece is the answer to the division.

Q. Add 6 + 7 on the number line.

A. 13. The expression 6 + 7 means start at 6, up 7, which brings you to 13 (see Figure 1-1): Figure 1-1:

Adding

6 + 7 = 13
on the 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20
number line.

Q. Subtract 12 – 4 on the number line.

A. 8. The expression 12 – 4 means start at 12, down 4, which brings you to 8 (see Figure 1-2): Figure 1-2:

Subtracting

12 – 4 = 8
on the 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20
number line.

05_288177-ch01.qxp 7/28/08 12:57 PM Page 13

Chapter 1: We’ve Got Your Numbers

13

Q. Multiply 2 · 5 on the number line.

A. 10. Starting at 0, count by twos a total of five times, which brings you to 10 (see Figure 1-3).

Figure 1-3:

Multiplying

2 · 5 = 10
on the 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20
number line.

Q. Divide 12 ÷ 4 on the number line.

A. 3. Block off the segment of the number line from 0 to 12. Now divide this segment evenly into three smaller pieces, as shown in Figure 1-4. Each of these pieces has a length of 4, so this is the answer to the problem.

Figure 1-4:

4

Dividing

12 ÷ 3 = 4
on the
number line. 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

9. Add the following numbers on the number

10. Subtract the following numbers on the
line:

number line:

a. 4 + 7 = ?

a. 10 – 6 = ?

b. 9 + 8 = ?

b. 14 – 9 = ?

c. 12 + 0 = ?

c. 18 – 18 = ?

d. 4 + 6 + 1 + 5 = ?

d. 9 – 3 + 7 – 2 + 1 = ?

Solve It

Solve It

11. Multiply the following numbers on the

12. Divide the following numbers on the
number line:

number line:

a. 2 · 7
d. 6 · 1
a. 8 ÷ 2 = ?

d. 10 ÷ 10 = ?

b. 7 · 2
e. 6 · 0
b. 15 ÷ 5 = ?

e. 7 ÷ 1 = ?

c. 4 · 3
f. 0 · 10
c. 18 ÷ 3 = ?

f. 0 ÷ 2 = ?

Solve It

Solve It

05_288177-ch01.qxp 7/28/08 12:57 PM Page 14

14 Part I: Back to Basics with Basic Math

The Column Lineup: Adding and Subtracting

To add or subtract large numbers, stack the numbers on top of each other so that all similar digits (ones, tens, hundreds, and so forth) form columns. Then work from right to left. Do the calculations vertically, starting with the ones column, then going to the tens column, and so forth:

ߜ When you’re adding and a column adds up to 10 or more, write down the ones digit of the result and carry the tens digit over to the column on the immediate left.

ߜ When you’re subtracting and the top digit in a column is less than the bottom digit, borrow from the column on the immediate left.

Q. Add 35 + 26 + 142.

Q. Subtract 843 – 91.

A. 203. Stack the numbers and add the

A. 752. Stack the numbers and subtract the
columns from right to left:

columns from right to left:

1

1 1

7

35

8

/43

26

− 91

142

+

752

203

When I try to subtract the tens column, 4

Notice that when I add the ones column
is less than 9, so I borrow 1 from the hun-

(5 + 6 + 2 = 13), I write the 3 below this
dreds column, changing the 8 to 7. Then
column and carry the 1 over to the

I place this 1 in front of the 4, changing it
tens column. Then, when I add the tens
to 14. Now I can subtract 14 – 9 = 5.

column (1 + 3 + 2 + 4 = 10), I write the 0
below this column and carry the 1 over
to the hundreds column.

13. Add 129 + 88 + 35 = ?

14. Find the following sum: 1,734 + 620 + 803 +

32 = ?

Solve It

Solve It

05_288177-ch01.qxp 7/28/08 12:58 PM Page 15

Chapter 1: We’ve Got Your Numbers

15

15. Subtract 419 – 57.

16. Subtract 41,024 – 1,786.

Solve It

Solve It

Multiplying Multiple Digits

To multiply large numbers, stack the first number on top of the second. Then multiply each digit of the bottom number, from right to left, by the top number. In other words, first multiply the top number by the ones digit of the bottom number. Then write down a 0 as a placeholder and multiply the top number by the tens digit of the bottom number. Continue the process, adding placeholders and multiplying the top number by the next digit in the bottom number.

When the result is a two-digit number, write down the ones digit and carry the tens digit to the next column. After multiplying the next two digits, add the number you carried over.

Add the results to obtain the final answer.

Q. Multiply 742 · 136.

Next, write down a 0 all the way to the
right in the row below the one that you

A. 100,912. Stack the first number on top of
just wrote. Multiply 3 by every number in
the second:

742, starting from the right and carrying
when necessary:

742

×136

1

742

Now multiply 6 by every number in 742,

136

×
starting from the right. Because 2 · 6 = 12,

4452
a two-digit number, you write down the 2

22260
and carry the 1 to the tens column. In the

Write down two 0s all the way to the
next column, you multiply 4 · 6 = 24 and
right of the row below the one that you
add the 1 you carried over, giving you a
just wrote. Repeat the process with 1:

total of 25. Write down the 5 and carry the

2 to the hundreds column. Multiply 7 · 6 =

742

42 and add the 2 you carried over, giving

136

×
you 44:

4452

22260

2 1

742

74200

136

×

4452

05_288177-ch01.qxp 7/28/08 12:58 PM Page 16

16 Part I: Back to Basics with Basic Math

To finish, add up the results:

742

136

×

4452

22260

74200

100912

So 742 · 136 = 100,912.

17. Multiply 75 · 42.

18. What’s 136 · 84?

Solve It

Solve It

19. Solve 1,728 · 405.

20. Multiply 8,912 · 767.

Solve It

Solve It

05_288177-ch01.qxp 7/28/08 12:58 PM Page 17

Chapter 1: We’ve Got Your Numbers

17

Cycling through Long Division

To divide larger numbers, use long division. Unlike the other Big Four operations, long division moves from left to right. For each digit in the divisor, the number you’re dividing, you complete a cycle of division, multiplication, and subtraction.

In some problems, the number at the very bottom of the problem isn’t a 0. In these cases, the answer has a remainder, which is a leftover piece that needs to be accounted for. In those cases, you write r followed by whatever number is left over.

Q. Divide 956 ÷ 4.

Subtract 15 – 12 to get 3. Then bring
down the next number (6) to make the

A. 239. Start off by writing the problem like
new number 36.

this:

23

)

4)

4 956

956

8

−

To begin, ask how many times 4 goes into

15

9 — that is, what’s 9 ÷ 4? The answer is 2

−12

(with a little left over), so write 2 directly

36
above the 9. Now multiply 2 · 4 to get 8,

Another cycle is complete, so begin the
place the answer directly below the 9,
next cycle by asking how many times 4
and draw a line beneath it:

goes into 36 — that is, what’s 36 ÷ 4? The

2
answer this time is 9. Write down 9 above
the 6, multiply 9 · 4 = 36, and place this

4)956
below the 36.

8

239

Subtract 9 – 8 to get 1. ( Note: After you

4) 956
subtract, the result should be less than

8

−
the divisor (in this problem, the divisor
is 4). Then bring down the next number

15

−12

(5) to make the new number 15.

36

36

2

4)956

Now subtract 36 – 36 = 0. Because you

−8
have no more numbers to bring down,

15
you’re finished, and the answer (that is,
the quotient) is the very top number

These steps are one complete cycle. To
of the problem:

complete the problem, you just need to
repeat them. Now ask how many times 4

239
goes into 15 — that is, what’s 15 ÷ 4? The

4)956
answer is 3 (with a little left over). So

−
write the 3 above the 5, and then multiply

8

3 · 4 to get 12. Write the answer under 15.

15

−12

23

36

− 36

4)956

0

− 8

15

−12

05_288177-ch01.qxp 7/28/08 12:58 PM Page 18

18 Part I: Back to Basics with Basic Math

Q. Divide 3,042 ÷ 5.

060

5)3042

A. 608 r 2. Start off by writing the problem

30

−
like this:

042

5)3042

Ask how many times 5 goes into 42 —
that is, what’s 42 ÷ 5? The answer is 8

To begin, ask how many times 5 goes into

(with a little bit left over), so complete

3. The answer is 0 — because 5 doesn’t
the cycle as follows:

go into 3 — so write a 0 above the 3. Now
you need to ask the same question using

0608

← quotient

5)

the first two digits of the divisor: How

3042

−30
many times does 5 go into 30 — that is,
s
what’s 30 ÷ 5? The answer is 6, so place

042

−40
the 6 over the 0. Here’s how to complete

2

← remainder
the first cycle:

06

Because you have no more numbers to

5)3042
bring down, you’re finished. The answer

30

−

(quotient) is at the top of the problem

04

(you can drop the leading 0), and the
remainder is at the bottom of the prob-

Next, ask how many times 5 goes into 4.

lem. So 3,042 ÷ 5 = 608 with a remainder

The answer is 0 — because 5 doesn’t go
of 2. To save space, write this answer as
into 4 — so write a 0 above the 4. Now

608 r 2.

bring down the next number (2), to make
the number 42:

21. Divide 741 ÷ 3.

22. Solve 3,245 ÷ 5.

Solve It

Solve It

23. Figure out 91,390 ÷ 8.

24. Find 792,541 ÷ 9.

Solve It

Solve It

05_288177-ch01.qxp 7/28/08 12:58 PM Page 19

Chapter 1: We’ve Got Your Numbers

19

Solutions to We’ve Got Your Numbers

The following are the answers to the practice questions presented in this chapter.

a Identify the ones, tens, hundreds, and thousands digit in the number 7,359.

a. 9 is the ones digit.

b. 5 is the tens digit.

c. 3 is the hundreds digit.

d. 7 is the thousands digit.

b 2,000 + 100 + 30 + 6 = 2,136

Millions

Hundred Thousands

Ten Thousands

Thousands

Hundreds

Tens

Ones

2

1

3

6
c 0 + 3,000 + 800 + 0 + 9 = 3,809. The first 0 is the leading zero, and the second 0 is the placeholder.

Millions

Hundred Thousands

Ten Thousands

Thousands

Hundreds

Tens

Ones

0

3

8

0

9
d 0 + 400,000 + 50,000 + 0 + 900 + 0 + 0 = 0,450,900. The first 0 is a leading zero, and the remaining three 0s are placeholders.

Millions

Hundred Thousands

Ten Thousands

Thousands

Hundreds

Tens

Ones

0

4

5

0

9

0

0
e Round to the nearest ten:

a. 29 → 30. The ones digit is 9, so round up.

b. 43 → 40. The ones digit is 3, so round down.

c. 75 → 80. The ones digit is 5, so round up.

d. 95 → 100. The ones digit is 5, so round up, rolling 9 over.

f Round to the nearest ten:

a. 164 → 160. The ones digit is 4, so round down.

b. 765 → 770. The ones digit is 5, so round up.

c. 1,989 → 1,990. The ones digit is 9, so round up.

d. 9,999,995 → 10,000,000. The ones digit is 5, so round up, rolling all of the 9s over.

05_288177-ch01.qxp 7/28/08 12:58 PM Page 20

20 Part I: Back to Basics with Basic Math
g Focus on the hundreds and tens digits to round to the nearest hundred.

a. 439 → 400. The tens digit is 3, so round down.

b. 562 → 600. The tens digit is 6, so round up.

c. 2,950 → 3,000. The tens digit is 5, so round up.

d. 109,974 → 110,000. The tens digit is 7, so round up, rolling over all the 9s.

h Focus on the thousands and hundreds digits to round to the nearest thousand.

a. 5,280 → 5,000. The hundreds digit is 2, so round down.

b. 77,777 → 78,000. The hundreds digit is 7, so round up.

c. 1,234,567 → 1,235,000. The hundreds digit is 5, so round up.

d. 1,899,999 → 1,900,000. The hundreds digit is 9, so round up, rolling over all the 9s to the left.

i Add on the number line.

a. 4 + 7 = 11. The expression 4 + 7 means start at 4, up 7, which brings you to 11.

b. 9 + 8 = 17. The expression 9 + 8 means start at 9, up 8, which brings you to 17.

c. 12 + 0 = 12. The expression 12 + 0 means start at 12, up 0, which brings you to 12.

d. 4 + 6 + 1 + 5 = 16. The expression 4 + 6 + 1 + 5 means start at 4, up 6, up 1, up 5, which brings you to 16.

j Subract on the number line.

a. 10 – 6 = 4. The expression 10 – 6 means start at 10, down 6, which brings you to 4.

b. 14 – 9 = 5. The expression 14 – 9 means start at 14, down 9, which brings you to 5.

c. 18 – 18 = 0. The expression 18 – 18 means start at 18, down 18, which brings you to 0.

d. 9 – 3 + 7 – 2 + 1 = 12. The expression 9 – 3 + 7 – 2 + 1 means start at 9, down 3, up 7, down 2, up 1, which brings you to 12.

k Multiply on the number line.

a. 2 · 7 = 14. Starting at 0, count by twos a total of seven times, which brings you to 14.

b. 7 · 2 = 14. Starting at 0, count by sevens a total of two times, which brings you to 14.

c. 4 · 3 = 12. Starting at 0, count by fours a total of three times, which brings you to 12.

d. 6 · 1 = 6. Starting at 0, count by sixes one time, which brings you to 6.

e. 6 · 0 = 0. Starting at 0, count by sixes zero times, which brings you to 0.

f. 0 · 10 = 0. Starting at 0, count by zeros a total of ten times, which brings you to 0.

l Divide on the number line.

a. 8 ÷ 2 = 4. Block off a segment of the number line from 0 to 8. Now divide this segment evenly into two smaller pieces. Each of these pieces has a length of 4, so this is the answer to the problem.

b. 15 ÷ 5 = 3. Block off a segment of the number line from 0 to 15. Divide this segment evenly into five smaller pieces. Each of these pieces has a length of 3, so this is the answer to the problem.

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Chapter 1: We’ve Got Your Numbers

21
c. 18 ÷ 3 = 6. Block off a segment of the number line from 0 to 18 and divide this segment evenly into three smaller pieces. Each piece has a length of 6, the answer to the problem.

d. 10 ÷ 10 = 1. Block off a segment of the number line from 0 to 10 and divide this segment evenly into ten smaller pieces. Each of these pieces has a length of 1.

e. 7 ÷ 1 = 7. Block off a segment of the number line from 0 to 7 and divide this segment evenly into 1 piece (that is, don’t divide it at all). This piece still has a length of 7.

f. 0 ÷ 2 = 0. Block off a segment of the number line from 0 to 0. The length of this segment is 0, so it can’t get any smaller. This shows you that 0 divided by any number is 0.

m 252
r 11,424

1 2

136

129

84

×

88

544

+35

10880

252

11424
n 3,189
s 699,840

2

1728

1734

405

620

×

803

8640

+ 32

691200

699840

3189
o 362
t 6,835,504

3 1

8912

419

× 767

− 57

62384

362

534720

6238400
p 39,238

6835504

3 1 0 9 11 1

4 1 0 2 4

−1 7 8 6
u 247

3 92 3 8

247

3)741
q

6

−

3,150

14

12

75

−

21

42

×

21

−

150

0

3000

3150

05_288177-ch01.qxp 7/28/08 12:58 PM Page 22

22 Part I: Back to Basics with Basic Math
v 649
x 88,060 r 1

0649

088060

5 3245

9

)

)792541

30

−

72

−

24

72

20

−

72

−

45

054

45

−

54

−

0

01

0

−

1
w 11,423 r 6

11423

8)91390

− 8

11

8

−

33

32

−

19

16

−

30

24

− 3

06_288177-ch02.qxp 7/28/08 12:58 PM Page 23

Chapter 2

Smooth Operators: Working with
the Big Four Operations

In This Chapter

ᮣ Rewriting equations using inverse operations and the commutative property

ᮣ Understanding the associative and distributive properties

ᮣ Working with inequalities such as >, <, ≠, and ≈

ᮣ Calculating powers and square roots

The Big Four operations (adding, subtracting, multiplying, and dividing) are basic stuff, but they’re really pretty versatile tools. In this chapter, I show you that the Big Four are really two pairs of inverse operations — that is, operations that undo each other. You also discover how the commutative property allows you to rearrange numbers in an expression.

And most importantly, you find out how to rewrite equations in alternative forms that allow you to solve problems more easily.

Next, I show you how to use parentheses to group numbers and operations together. You discover how the associative property ensures that, in certain cases, the placement of parentheses doesn’t change the answer to a problem. You also work with four types of inequalities:

>, <, ≠, and ≈. Finally, I show you that raising a number to a power is a shortcut for multiplication and explain how to find the square root of a number.

Switching Things Up with Inverse Operations
and the Commutative Property

The Big Four operations are actually two pairs of inverse operations, which means the operations can undo each other:

ߜ Addition and subtraction: Subtraction undoes addition. For example, if you start with 3 and add 4, you get 7. Then, when you subtract 4, you undo the original addition and arrive back at 3:

3 + 4 = 7

→ 7 – 4 = 3

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24 Part I: Back to Basics with Basic Math

This idea of inverse operations makes a lot of sense when you look at the number line.

On a number line, 3 + 4 means start at 3, up 4. And 7 – 4 means start at 7, down 4. So when you add 4 and then subtract 4, you end up back where you started.

ߜ Multiplication and division: Division undoes multiplication. For example, if you start with 6 and multiply by 2, you get 12. Then, when you divide by 2, you undo the original multiplication and arrive back at 6:

6 · 2 = 12

→

12 ÷ 2 = 6

The commutative property of addition tells you that you can change the order of the numbers in an addition problem without changing the result, and the commutative property of multiplication says you can change the order of the numbers in a multiplication problem without changing the result. For example,

2 + 5 = 7

→

5 + 2 = 7

3 · 4 = 12

→

4 · 3 = 12

Through the commutative property and inverse operations, every equation has four alternative forms that contain the same information expressed in slightly different ways. For example, 2 + 3 = 5 and 3 + 2 = 5 are alternative forms of the same equation but tweaked using the commutative property. And 5 – 3 = 2 is the inverse of 2 + 3 = 5. Finally, 5 – 2 = 3 is the inverse of 3 + 2 = 5.

You can use alternative forms of equations to solve fill-in-the-blank problems. As long as you know two numbers in an equation, you can always find the remaining number. Just figure out a way to get the blank to the other side of the equal sign:

ߜ When the first number is missing in any problem, use the inverse to turn the problem around:

_______________ + 6 = 10

→ 10 – 6 = _______________

ߜ When the second number is missing in an addition or multiplication problem, use the commutative property and then the inverse:

9 + _______________ = 17

→ _______________ + 9 = 17 → 17 – 9 = _______________

ߜ When the second number is missing in a subtraction or multiplication problem, just switch around the two numbers that are next to the equal sign:

15 – _______________ = 8 →

15 – 8 = _______________

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Chapter 2: Smooth Operators: Working with the Big Four Operations

25

Q. What’s the inverse equation to 16 – 9 = 7?

Q. Fill in the blank: _______________ ÷ 3 = 13.

A. 7 + 9 = 16. In the equation 16 – 9 = 7, you

A. 39. Use inverse operations to turn the
start at 16 and subtract 9, which brings
problem from division to multiplication:

you to 7. The inverse equation undoes

_______________ ÷ 3 = 13

→
this process, so you start at 7 and add 9,
which brings you back to 16:

13 · 3 = _______________

16 – 9 = 7

→ 7 + 9 = 16

Now you can solve the problem by

Q.

multiplying 13 · 3 = 39.

What’s the inverse equation to 6 · 7 = 42?

Q.

A.

Solve this problem by filling in the blank:

42 ÷ 7 = 6. In the equation 6 · 7 = 42, you

16 + _______________ = 47.

start at 6 and multiply by 7, which brings
you to 42. The inverse equation undoes

A. 31. First, use the commutative property
this process, so you start at 42 and
to reverse the addition:

divide by 7, which brings you back to 6:

16 + _______________ = 47

→

6 · 7 = 42

→ 42 ÷ 7 = 6

_______________ + 16 = 47

Q. Use inverse operations and the commu-

Now use inverse operations to change
tative property to find three alternative
the problem from addition to subtraction:

forms of the equation 7 – 2 = 5.

_______________ + 16 = 47

→

A. 5 + 2 = 7, 2 + 5 = 7, and 7 – 5 = 2. First,

47 – 16 = _______________
use inverse operations to change sub-
traction to addition:

At this point, you can solve the problem

7 – 2 = 5

→ 5 + 2 = 7
by subtracting 47 – 16 = 31.

Now use the commutative property to

Q. Fill in the blank: 64 – _______________ = 15.

change the order of this addition:

A. 49. Switch around the last two numbers

5 + 2 = 7

→ 2 + 5 = 7
in the problem:

Finally, use inverse operations to change

64 – _______________ = 15

→
addition to subtraction:

64 – 15 = _______________

2 + 5 = 7

→ 7 – 5 = 2

Now you can solve the problem by sub-
tracting 64 – 15 = 49.

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26 Part I: Back to Basics with Basic Math

1. Using inverse operations, write down an

2. Use the commutative property to write
alternative form of each equation:

down an alternative form of each equation:

a. 8 + 9 = 17
a. 19 + 35 = 54
b. 23 – 13 = 10
b. 175 + 88 = 263
c. 15 · 5 = 75
c. 22 · 8 = 176
d. 132 ÷ 11 = 12
d. 101 · 99 = 9,999

Solve It

Solve It

3. Use inverse operations and the commuta-

4. Fill in the blank in each equation:

tive property to find all three alternative
a. _______________ – 74 = 36
forms for each equation:

b. _______________ · 7 = 105
a. 7 + 3 = 10
c. 45 + _______________ = 132
b. 12 – 4 = 8
d. 273 – _______________ = 70
c. 6 · 5 = 30
e. 8 · _______________ = 648
d. 18 ÷ 2 = 9
f. 180 ÷ _______________ = 9

Solve It

Solve It

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Chapter 2: Smooth Operators: Working with the Big Four Operations

27

Getting with the In-Group: Parentheses
and the Associative Property

Parentheses group operations together, telling you to do any operations inside a set of parentheses before you do operations outside of it. Parentheses can make a big difference in the result you get when solving a problem, especially in a problem with mixed operations. In two important cases, however, moving parentheses doesn’t change the answer to a problem: ߜ The associative property of addition says that when every operation is addition, you can group numbers however you like and choose which pair of numbers to add first; you can move parentheses without changing the answer.

ߜ The associative property of multiplication says you can choose which pair of numbers to multiply first, so when every operation is multiplication, you can move parentheses without changing the answer.

Taken together, the associative property and the commutative property (which I discuss in the preceding section) allow you to completely rearrange all the numbers in any problem that’s either all addition or all multiplication.

Q. What’s (21 – 6) ÷ 3? What’s 21 – (6 ÷ 3)?

Q. Solve 1 + (9 + 2) and (1 + 9) + 2.

A. 5 and 19. To solve (21 – 6) ÷ 3, first do

A. 12 and 12. To solve 1 + (9 + 2), first do
the operation inside the parentheses —
the operation inside the parentheses —
that is, 21 – 6 = 15:

that is, 9 + 2 = 11:

(21 – 6) ÷ 3 = 15 ÷ 3

1 + (9 + 2) = 1 + 11

Now finish the problem by dividing:

Finish up by adding 1 + 11 = 12.

15 ÷ 3 = 5.

To solve (1 + 9) + 2, first do the operation

To solve 21 – (6 ÷ 3), first do the opera-
inside the parentheses — that is,
tion inside the parentheses — that is,

1 + 9 = 10:

6 ÷ 3 = 2:

(1 + 9) + 2 = 10 + 2

21 – (6 ÷ 3) = 21 – 2

Finish up by adding 10 + 2 = 12. Notice

Finish up by subtracting 21 – 2 = 19.

that the only difference between the two

Notice that the placement of the paren-
problems is the placement of the paren-
theses changes the answer.

theses, but because both operations are
addition, moving the parentheses doesn’t
change the answer.

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28 Part I: Back to Basics with Basic Math

Q. Solve 2 · (4 · 3) and (2 · 4) · 3.

Q. Solve 41 · 5 · 2.

A. 24 and 24. To solve 2 · (4 · 3), first do the

A. 410. The last two numbers are small, so
operation inside the parentheses — that
place parentheses around these numbers:

is, 4 · 3 = 12:

41 · 5 · 2 = 41 · (5 · 2)

2 · (4 · 3) = 2 · 12

First, do the multiplication inside the

Finish by multiplying 2 · 12 = 24.

parentheses:

41 · (5 · 2) = 41 · 10

To solve (2 · 4) · 3, first do the operation
inside the parentheses — that is, 2 · 4 = 8:

Now you can easily multiply 41 · 10 = 410.

(2 · 4) · 3 = 8 · 3

Finish by multiplying 8 · 3 = 24. No matter
how you group the multiplication, the
answer is the same.

5. Find the value of (8 · 6) + 10.

6. Find the value of 123 ÷ (145 – 144).

Solve It

Solve It

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Chapter 2: Smooth Operators: Working with the Big Four Operations

29

7. Solve the following two problems:

8. Solve the following two problems:

a. (40 ÷ 2) + 6 = ?

a. (16 + 24) + 19
b. 40 ÷ (2 + 6) = ?

b. 16 + (24 + 19)

Do the parentheses make a difference in

Do the parentheses make a difference in
the answers?

the answers?

Solve It

Solve It

9. Solve the following two problems:

10. Find the value of 93,769 · 2 · 5. ( Hint: Use
the associative property for multiplication
a. (18 · 25) · 4
to make the problem easier.)

b. 18 · (25 · 4)

Solve It

Do the parentheses make a difference in
the answers?

Solve It

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30 Part I: Back to Basics with Basic Math

Becoming Unbalanced: Inequalities

When numbers aren’t equal in value, you can’t use the equal sign (=) to turn them into an equation. Instead, you use a variety of other symbols to turn them into an inequality: ߜ The symbol > means is greater than, and the symbol < means is less than: 6 > 3 means 6 is greater than 3

7 < 10 means 7 is less than 10

If you’re not sure whether to use > or <, remember that the big open mouth of the symbol always faces the larger number. For example, 5 < 7, but 7 > 5.

ߜ The symbol ≠ means doesn’t equal. It’s not as useful as > or < because it doesn’t tell you whether a number is greater than or less than another number. Mostly, ≠ points out a mistake or inaccuracy in a pre-algebra calculation.

ߜ The symbol ≈ means approximately equals. You use it when you’re rounding numbers and estimating solutions to problems — that is, when you’re looking for an answer that’s close enough but not exact. The symbol ≈ allows you to make small adjustments to numbers to make your work easier. (See Chapter 1 for more on estimating and rounding.) Q. Place the correct symbol (=, >, or <) in

Q. Find an approximate solution to
the blank: 2 + 2 _______________ 5.

2,000,398 + 6,001,756.

A. 2 + 2 < 5. Because 2 + 2 = 4 and 4 is less

A. 8,000,000. The two numbers are both in
than 5, use the symbol that means is
the millions, so you can use ≈ to round
less than.

them to the nearest million:

2,000,398 + 6,001,756 ≈ 2,000,000 +

Q. Place the correct symbol (=, >, or <) in

6,000,000
the blank: 42 – 19 _______________ 5 · 4.

Now it’s easy to add 2,000,000 + 6,000,000 =

A. 42 – 19 > 5 · 4. Because 42 – 19 = 23 and

8,000,000.

5 · 4 = 20, and 23 is greater than 20, use
the symbol that means is greater than.

Q. Sam worked 7 hours for his parents at $8
an hour, and his parents paid him with a

$50 bill. Use the symbol ≠ to point out
why Sam was upset.

A. $50 ≠ $56. Sam worked 7 hours for $8 an
hour, so here’s how much he earned:

7 · $8 = $56

He was upset because his parents didn’t
pay him the correct amount: $50 ≠ $56.

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Chapter 2: Smooth Operators: Working with the Big Four Operations

31

11. Place the correct symbol (=, >, or <) in the

12. Change the ≠ signs to either > or <:

blanks:

a. 17 + 14 ≠ 33
a. 4 + 6 _______________ 13
b. 144 – 90 ≠ 66
b. 9 · 7 _______________ 62
c. 11 · 14 ≠ 98
c. 33 – 16 _______________ 60 ÷ 3
d. 150 ÷ 6 ≠ 20
d. 100 ÷ 5 _______________ 83 – 63

Solve It

Solve It

13. Tim’s boss paid him for 40 hours of work

14. Find an approximate solution to 10,002 –
last week. Tim accounted for his time by

6,007.

saying that he spent 19 hours with clients,

11 hours driving, and 7 hours doing paper-

Solve It
work. Use ≠ to show why Tim’s boss was
unhappy with Tim’s work.

Solve It

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32 Part I: Back to Basics with Basic Math

Special Times: Powers and Square Roots

Raising a number to a power is a quick way to multiply a number by itself. For example, 25, which you read as two to the fifth power, means that you multiply 2 by itself 5 times: 25 = 2 · 2 · 2 · 2 · 2 = 32

The number 2 is called the base, and the number 5 is called the exponent.

Powers of ten — that is, powers with 10 in the base — are especially important because the number system is based on them. Fortunately, they’re very easy to work with. To raise 10 to the power of any positive whole number, write down the number 1 followed by the number of 0s indicated by the exponent. For example, 103 is 1,000.

Here are some important rules for finding powers that contain 0 or 1:

ߜ Every number raised to the power of 1 equals that number itself.

ߜ Every number (except 0) raised to the power of 0 is equal to 1. For example, 100 is 1 followed by no 0s — that is, 1.

ߜ The number 0 raised to the power of any number (except 0) equals 0, because no matter how many times you multiply 0 by itself, the result is 0.

Mathematicians have decided that 00 is undefined — that is, it doesn’t equal any number.

ߜ The number 1 raised to the power of any number equals 1, because no matter how many times you multiply 1 by itself, the result is 1.

When you multiply any number by itself, the result is a square number. So when you raise any number to the power of two, you’re squaring that number. For example, here’s 52, or five squared:

52 = 5 · 5 = 25

The inverse of squaring a number is called finding the square root of a number (inverse operations undo each other — see the earlier section “Switching Things Up with Inverse Operations and the Commutative Property”). When you find the square root of a number, you discover a new number which, multiplied by itself, equals the number you started with.

For example, here’s the square root of 25:

25 = 5 (because 5 · 5 = 25)

Q. What is 34?

Q. What is 106?

A. 81. The expression 34 tells you to

A. 1,000,000. Using the power of ten rule,
multiply 3 by itself 4 times:

106 is 1 followed by six 0s, so 106 =

1,000,000.

3 · 3 · 3 · 3 = 81

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Chapter 2: Smooth Operators: Working with the Big Four Operations

33

Q. What is 36 ?

Q. What is 256 ?

A. 6. To find 36 , you want to find a

A. 16. To find 256 , you want to find a
number that, when multiplied by itself,
number that, when multiplied by itself,
equals 36. You know that 6 · 6 = 36, so
equals 256. Try guessing to narrow down

36 = 6.

the possibilities. Start by guessing 10:

10 · 10 = 100

256 > 100, so 256 is greater than 10.

Guess 20:

20 · 20 = 400

256 < 400, so 256 is between 10 and 20.

Guess 15:

15 · 15 = 225

256 > 225, so 256 is between 15 and 20.

Guess 16:

16 · 16 = 256

This is correct, so 256 = 16.

15. Find the value of the following powers:

16. Find the value of the following powers:

a. 62
a. 104
b. 35
b. 1010
c. 27
c. 1015
d. 28 ( Hint: You can make your work easier
d. 101
by using the answer to c. )

Solve It

Solve It

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34 Part I: Back to Basics with Basic Math

Answers to Problems in Smooth Operators

The following are the answers to the practice questions presented in this chapter.

a Using inverse operations, write down an alternative form of each equation: a. 8 + 9 = 17: 17 – 9 = 8
b. 23 – 13 = 10: 10 + 13 = 23
c. 15 · 5 = 75: 75 ÷ 5 = 15
d. 132 ÷ 11 = 12: 12 · 11 = 132
b Use the commutative property to write down an alternative form of each equation: a. 19 + 35 = 54: 35 + 19 = 54
b. 175 + 88 = 263: 88 + 175 = 263
c. 22 · 8 = 176: 8 · 22 = 176
d. 101 · 99 = 9,999: 99 · 101 = 9,999
c Use inverse operations and the commutative property to find all three alternative forms for each equation:

a. 7 + 3 = 10: 10 – 3 = 7, 3 + 7 = 10, and 10 – 7 = 3
b. 12 – 4 = 8: 8 + 4 = 12, 4 + 8 = 12, and 12 – 8 = 4
c. 6 · 5 = 30: 30 ÷ 5 = 6, 5 · 6 = 30, and 30 ÷ 6 = 5
d. 18 ÷ 2 = 9: 9 · 2 = 18, 2 · 9 = 18, and 18 ÷ 9 = 2
d Fill in the blank in each equation:

a. 110. Rewrite _______________ – 74 = 36 as its inverse:

36 + 74 = _______________

Therefore, 36 + 74 = 110.

b. 15. Rewrite _______________ · 7 = 105 as its inverse:

105 ÷ 7 = _______________

So 105 ÷ 7 = 15.

c. 87. Rewrite 45 + _______________ = 132 using the commutative property: _______________ + 45 = 132

Now rewrite this equation as its inverse:

132 – 45 = _______________

Therefore, 132 – 45 = 87.

d. 203. Rewrite 273 – _______________ = 70 by switching around the two numbers next to the equal sign:

273 – 70 = _______________

So 273 – 70 = 203.

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Chapter 2: Smooth Operators: Working with the Big Four Operations

35
e. 81. Rewrite 8 · _______________ = 648 using the commutative property: _______________ · 8 = 648

Now rewrite this equation as its inverse:

648 ÷ 8 = _______________

So 648 ÷ 8 = 81.

f. 20. Rewrite 180 ÷ _______________ = 9 by switching around the two numbers next to the equal sign:

180 ÷ 9 = _______________

So 180 ÷ 9 = 20.

e 58. First, do the multiplication inside the parentheses:

(8 · 6) + 10 = 48 + 10

Now add: 48 + 10 = 58.

f 123. First, do the subtraction inside the parentheses:

123 ÷ (145 – 144) = 123 ÷ 1

Now simply divide 123 ÷ 1 = 123.

g Solve the following two problems:

a. (40 ÷ 2) + 6 = 20 + 6 = 26
b. 40 ÷ (2 + 6) = 40 ÷ 8 = 5

Yes, the placement of parentheses changes the result.

h Solve the following two problems:

a. (16 + 24) + 19 = 40 + 19 = 59
b. 16 + (24 + 19) = 16 + 43 = 59

No, because of the associative property of addition, the placement of parentheses doesn’t change the result.

i Solve the following two problems:

a. (18 · 25) · 4 = 450 · 4 = 1,800
b. 18 · (25 · 4) = 18 · 100 = 1,800

No, because of the associative property of multiplication, the placement of parentheses doesn’t change the result.

j 93,769 · 2 · 5 = 937,690. The problem is easiest to solve by placing parentheses around 2 · 5: 93,769 · (2 · 5) = 93,769 · 10 = 937,690

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36 Part I: Back to Basics with Basic Math
k Place the correct symbol (=, >, or <) in the blanks:

a. 4 + 6 = 10, and 10 < 13
b. 9 · 7 = 63, and 63 > 62
c. 33 – 16 = 17 and 60 ÷ 3 = 20, so 17 < 20.

d. 100 ÷ 5 = 20 and 83 – 63 = 20, so 20 = 20.

l Change the ≠ signs to either > or <:

a. 17 + 14 = 31, and 31 < 33
b. 144 – 90 = 54, and 54 < 66
c. 11 · 14 = 154, and 154 > 98
d. 150 ÷ 6 = 25, and 25 > 20
m 19 + 11 + 7 = 37 ≠ 40
n 10,002 – 6,007 ≈ 4,000
o Find the value of the following powers:

a. 62 = 6 · 6 = 36.

b. 35 = 3 · 3 · 3 · 3 · 3 = 243.

c. 27 = 2 · 2 · 2 · 2 · 2 · 2 · 2 = 128.

d. 28 = 2 · 2 · 2 · 2 · 2 · 2 · 2 · 2 = 256. You already know from part c that 27 = 128, so multiply this number by 2 to get your answer: 128 · 2 = 256.

p Find the value of the following powers:

a. 104 = 10,000. Write 1 followed by four 0s.

b. 1010 = 10,000,000,000. Write 1 followed by ten 0s.

c. 1015 = 1,000,000,000,000,000. Write 1 followed by fifteen 0s.

d. 101 = 10. Any number raised to the power of 1 is that number.

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Chapter 3

Getting Down with Negative Numbers

In This Chapter

ᮣ Understanding negative numbers

ᮣ Finding the absolute value of a number

ᮣ Adding and subtracting negative numbers

ᮣ Multiplying and dividing negative numbers

N egative numbers, which are commonly used to represent debt and really cold temperatures, represent amounts less than zero. Such numbers arise when you subtract a large number from a smaller one. In this chapter, you apply the Big Four operations (adding, subtracting, multiplying, and dividing) to negative numbers.

Understanding Where Negative

Numbers Come From

When you first discovered subtraction, you were probably told that you can’t take a small number minus a greater number. For example, if you start with four marbles, you can’t subtract six because you can’t take away more marbles than you have. This rule is true for marbles, but in other situations, you can subtract a big number from a small one. For example, if you have $4 and you buy something that costs $6, you end up with less than $0 dollars —
that is, –$2, which means a debt of $2.

A number with a minus sign in front of it, like –2, is called a negative number. You call the number –2 either negative two or minus two. Negative numbers appear on the number line to the left of 0, as shown in Figure 3-1.

Figure 3-1:

Negative
numbers
on the

–8

–7

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

7

8
number line.

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38 Part I: Back to Basics with Basic Math

Subtracting a large number from a small number makes sense on the number line. Just use the rule for subtraction that I show you in Chapter 2: Start at the first number and count left the second number of places.

When you don’t have a number line to work with, here’s a simple rule for subtracting a small number minus a big number: Switch the two numbers around and take the big number minus the small one; then attach a negative sign to the result.

Q. Use the number line to subtract 5 – 8.

A. –3. On the number line, 5 – 8 means start at 5, left 8.

–8

–7

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

7

8

Q. What is 11 – 19?

A. –8. Because 11 is less than 19, subtract 19 – 11, which equals 8, and attach a minus sign to the result. Therefore, 11 – 19 = –8.

1. Using the number line, subtract the

2. Find the answers to the following
following numbers:

subtraction problems:

a. 1 – 4 = ________________
a. 15 – 22 = ________________
b. 3 – 7 = ________________
b. 27 – 41 = ________________
c. 6 – 8 = ________________
c. 89 – 133 = ________________
d. 7 – 14 = ________________
d. 1,000 – 1,234 = ________________

Solve It

Solve It

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Chapter 3: Getting Down with Negative Numbers

39

Sign-Switching: Understanding Negation
and Absolute Value

When you attach a minus sign to any number, you negate that number. Negating a number means changing its sign to the opposite sign, so

ߜ Attaching a minus sign to a positive number makes it negative.

ߜ Attaching a minus sign to a negative number makes it positive. The two adjacent (side by side) minus signs cancel each other out.

ߜ Attaching a minus sign to 0 doesn’t change its value, so –0 = 0.

In contrast to negation, placing two bars around a number gives you the absolute value of that number. Absolute value is the positive value of a number, regardless of whether you started out with a negative or positive number. It’s kind of like the number’s distance from 0
on the number line:

ߜ The absolute value of a positive number is the same number.

ߜ The absolute value of a negative number makes it a positive number.

ߜ Placing absolute value bars around 0 doesn’t change its value, so |0| = 0.

ߜ Placing a minus sign outside absolute value bars gives you a negative result — for example, –|6| = –6, and –|–6| = –6.

Q. Negate the number 7.

Q. What number does |9| equal?

A. –7. Negate 7 by attaching a negative sign

A. 9. The number 9 is already positive, so
to it: –7.

the absolute value of 9 is also 9.

Q. Find the negation of –3.

Q. What number does |–17| equal?

A. 3. The negation of –3 is – (–3). The two

A. 17. Because –17 is negative, the absolute
adjacent minus signs cancel out, which
value of –17 is 17.

gives you 3.

Q. Solve this absolute value problem: –|9 –

Q. What’s the negation of 7 – 12?

13| = ?

A. 5. First do the subtraction, which tells

A. –4. Do the subtraction first: 9 – 13 = –4,
you 7 – 12 = –5. To find the negation of
which is negative, so the absolute value

–5, attach a minus sign to the answer:

of –4 is 4. But the minus sign negates this

–(–5). The two adjacent minus signs
result, so the answer is –4.

cancel out, which gives you 5.

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40 Part I: Back to Basics with Basic Math

3. Negate each of the following numbers and

4. Solve the following absolute value
expressions by attaching a minus sign and
problems:

then canceling out minus signs when
a. |7| = ?

possible:

b. |–11| = ?

a. 6
c. |3 + 15| = ?

b. –29
d. –|10 – 1| = ?

c. 0
e. |1 – 10| = ?

d. 10 + 4
f. |0| = ?

e. 15 – 7
f. 9 – 10

Solve It

Solve It

Adding with Negative Numbers

When you understand what negative numbers mean, you can add them just like the positive numbers you’re used to. The number line can help make sense of this. You can turn every problem into a sequence of ups and downs, as I show you in Chapter 1. When you’re adding on the number line, starting with a negative number isn’t much different from starting with a positive number.

Adding a negative number is the same as subtracting a positive number — that is, go down (to the left) on the number line. This rule works regardless of whether you start with a positive or negative number.

After you understand how to add negative numbers on the number line, you’re ready to work without the number line. This becomes important when numbers get too large to fit on the number line. Here are some tricks:

ߜ Adding a negative number plus a positive number: Switch around the two numbers (and their signs), turning the problem into subtraction.

ߜ Adding a positive number plus a negative number: Switch around the plus sign and the minus sign, turning the problem into subtraction.

ߜ Adding two negative numbers: Drop both minus signs and add the numbers as if they were both positive; then attach a minus sign to the result.

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Chapter 3: Getting Down with Negative Numbers

41

Q. Use the number line to add –3 + 5.

A. 2. On the number line, –3 + 5 means start at –3, up 5, which brings you to 2:

–8

–7

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

7

8

Q. Use the number line to add 6 + –2.

A. 4. On the number line, 6 + –2 means start at 6, down 2, which brings you to 4:

–8

–7

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

7

8

Q. Use the number line to add –3 + –4.

A. –7. On the number line, –3 + –4 means start at –3, down 4, which brings you to –7:

–8

–7

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

7

8

Q. Add –23 + 39.

A. 16. Switch around the two numbers with the signs attached:

–23 + 39 = + 39 – 23

Now you can drop the plus sign and use the minus sign for subtraction:

39 – 23 = 16

5. Use the number line to solve the following

6. Solve the following addition problems
addition problems:

without using the number line:

a. –5 + 6
a. –17 + 35
b. –1 + –7
b. 29 + –38
c. 4 + –6
c. –61 + –18
d. –3 + 9
d. 70 + –63
e. 2 + –1
e. –112 + 84
f. –4 + –4
f. –215 + –322

Solve It

Solve It

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42 Part I: Back to Basics with Basic Math

Subtracting with Negative Numbers

Subtracting a negative number is the same as adding a positive number — that is, go up on the number line. This rule works regardless of whether you start with a positive or negative number.

When subtracting a negative number, remember that the two back-to-back minus signs cancel each other out, leaving you with a plus sign. (Kind of like when you insist you can’t not laugh at your friends, because they’re really pretty ridiculous; the two negatives mean you have to laugh, which is a positive statement.)

Note: Math books often put parentheses around the negative number you’re subtracting so the signs don’t run together, so 3 – –5 is the same as 3 – (–5).

When taking a negative number minus a positive number, drop both minus signs and add the two numbers as if they were both positive; then attach a minus sign to the result.

Q. Use the number line to subtract –1 – 4.

A. –5. On the number line, –1 – 4 means start at –1, down 4, which brings you to –5:

–8

–7

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

7

8

7. Use the number line to solve the following

8. Solve the following subtraction problems
subtraction problems:

without using the number line:

a. –3 – 4
a. 17 – (–26)

b. 5 – (–3)

b. –21 – 45
c. –1 – (–8)

c. –42 – (–88)

d. –2 – 4
d. –67 – 91
e. –4 – 2
e. 75 – (–49)

f. –6 – (–10)

f. –150 – (–79)

Solve It

Solve It

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Chapter 3: Getting Down with Negative Numbers

43

Comparing Signs of the Times(And Division)

of Negative Numbers

Multiplying and dividing negative numbers is basically the same as it is with positive numbers. The presence of one or more minus signs (–) doesn’t change the numerical part of the answer. The only question is whether the sign of the answer is positive or negative.

Remember the following when you multiply or divide negative numbers:

ߜ If the two numbers have the same sign, the result is always positive.

ߜ If the two numbers have opposite signs, the result is always negative.

Q. Solve the following four multiplication

Q. Solve the following four division problems:

problems:

a. 18 ÷ 3 = ________________
a. 5 · 6 = ________________
b. –18 ÷ 3 = ________________
b. –5 · 6 = ________________
c. 18 ÷ –3 = ________________
c. 5 · –6 = ________________
d. –18 ÷ –3 = ________________
d. –5 · –6 = ________________

A. The numerical part of the answer (6)

A. As you can see from this example, the
doesn’t change. Only the sign of the
numerical part of the answer (30) doesn’t
answer changes, depending on the signs
change. Only the sign of the answer
of the two numbers in the problem.

changes, depending on the signs of the
a. 18 ÷ 3 = 6
two numbers in the problem.

b. –18 ÷ 3 = –6
a. 5 · 6 = 30
c. 18 ÷ –3 = –6
b. –5 · 6 = –30
d. –18 ÷ –3 = 6
c. 5 · –6 = –30
d. –5 · –6 = 30

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44 Part I: Back to Basics with Basic Math

Q. What is –84 · 21?

Q. What is –580 ÷ –20?

A. –1,764. First, drop the signs and

A. 29. Drop the signs and divide (you can
multiply:

use long division, as I show you in

Chapter 1):

84 · 21 = 1,764

580 ÷ 20 = 29

The numbers –84 and 21 have different
signs, so the answer is negative: –1,764.

The numbers –580 and –20 have the
same signs, so the answer is positive: 29.

9. Solve the following multiplication problems:

10. Solve the following division problems:

a. 7 · 11 = ________________
a. 32 ÷ –8 = ________________
b. –7 · 11 = ________________
b. –32 ÷ –8 = ________________
c. 7 · –11 = ________________
c. –32 ÷ 8 = ________________
d. –7 · –11 = ________________
d. 32 ÷ 8 = ________________

Solve It

Solve It

11. What is –65 · 23?

12. Find –143 · –77.

Solve It

Solve It

13. Calculate 216 ÷ –9.

14. What is –3,375 ÷ –25?

Solve It

Solve It

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Chapter 3: Getting Down with Negative Numbers

45

Answers to Problems in Getting Down with

Negative Numbers

The following are the answers to the practice questions presented in this chapter.

a Using the number line, subtract the following numbers:

a. 1 – 4 = –3. Start at 1, down 4.

b. 3 – 7 = –4. Start at 3, down 7.

c. 6 – 8 = –2. Start at 6, down 8.

d. 7 – 14 = –7. Start at 7, down 14.

b Find the answers to the following subtraction problems:

a. 15 – 22 = –7. Fifteen is less than 22, so subtract 22 – 15 = 7 and attach a minus sign to the result: –7.

b. 27 – 41 = –14. Twenty-seven is less than 41, so subtract 41 – 27 = 14 and attach a minus sign to the result: –14.

c. 89 – 133 = –44. Eighty-nine is less than 133, so subtract 133 – 89 = 44 and attach a minus sign to the result: –44.

d. 1,000 – 1,234 = –234. One thousand is less than 1,234, so subtract 1,234 – 1,000 = 234 and attach a minus sign to the result: –234.

c Negate each of the following numbers and expressions by attaching a minus sign and then canceling out minus signs when possible:

a. –6. To negate 6, attach a minus sign: –6.

b. 29. To negate –29, attach a minus sign: –(–29). Now cancel adjacent minus signs: –(–29) = 29.

c. 0. Zero is its own negation.

d. –14. Add first: 10 + 4 = 14, and the negation of 14 is –14.

e. –8. Subtract first: 15 – 7 = 8, and the negation of 8 is –8.

f. 1. Begin by subtracting: 9 – 10 = –1, and the negation of –1 is 1.

d Solve the following absolute value problems:

a. |7| = 7. Seven is already positive, so the absolute value of 7 is also 7.

b. |–11| = 11. The number –11 is negative, so the absolute value of –11 is 11.

c. |3 + 15| = 18. First, do the addition inside the absolute value bars: 3 + 15 = 18, which is positive. The absolute value of 18 is 18.

d. –|10 – 1| = –9. First do the subtraction: 10 – 1 = 9, which is positive. The absolute value of 9 is 9. You have a negative sign outside the absolute value bars, so negate your answer to get –9.

e. |1 – 10| = 9. Begin by subtracting: 1 – 10 = –9, which is negative. The absolute value of –9 is 9.

f. |0| = 0. The absolute value of 0 is 0.

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46 Part I: Back to Basics with Basic Math
e Use the number line to solve the following addition problems:

a. –5 + 6 = 1. Start at –5, go up 6.

b. –1 + –7 = –8. Start at –1, go down 7.

c. 4 + –6 = –2. Start at 4, go down 6.

d. –3 + 9 = 6. Start at –3, go up 9.

e. 2 + –1 = 1. Start at 2, go down 1.

f. –4 + –4 = –8. Start at –4, go down 4.

f Solve the following addition problems without using the number line:

a. –17 + 35 = 18. Switch around the numbers (with their signs) to turn the problem into subtraction:

–17 + 35 = 35 – 17 = 18
b. 29 + –38 = –9. Switch around the signs to turn the problem into subtraction: 29 + –38 = 29 – 38 = –9
c. –61 + –18 = –79. Drop the signs, add the numbers, and negate the result: 61 + 18 = 79, so –61 + –18 = –79.

d. 70 + –63 = 9. Turn the problem into subtraction:

70 + –63 = 70 – 63 = 9
e. –112 + 84 = –28. Turn the problem into subtraction:

–112 + 84 = 84 – 112 = –28
f. –215 + –322 = –537. Drop the signs, add the numbers, and negate the result: 215 + 322 = 537, so –215 + –322 = –537
g Use the number line to solve the following subtraction problems:

a. –3 – 4 = –7. Start at –3, down 4.

b. 5 – (–3) = 8. Start at 5, up 3.

c. –1 – (–8) = 7. Start at –1, up 8.

d. –2 – 4 = –6. Start at –2, down 4.

e. –4 – 2 = –6. Start at –4, down 2.

f. –6 – (–10) = 4. Start at –6, up 10.

h Solve the following subtraction problems without using the number line:

a. 17 – (–26) = 43. Cancel the adjacent minus signs to turn the problem into addition: 17 – (–26) = 17 + 26 = 43
b. –21 – 45 = –66. Drop the signs, add the numbers, and negate the result: 21 + 45 = 66, so –21 – 45 = –66

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Chapter 3: Getting Down with Negative Numbers

47
c. –42 – (–88) = 46. Cancel the adjacent minus signs to turn the problem into addition:

–42 – (–88) = –42 + 88

Now switch around the numbers (with their signs) to turn the problem back into subtraction: 88 – 42 = 46
d. –67 – 91 = –158. Drop the signs, add the numbers, and negate the result: 67 + 91 = 158, so –67 – 91 = –158
e. 75 – (–49) = 124. Cancel the adjacent minus signs to turn the problem into addition: 75 – (–49) = 75 + 49 = 124
f. –150 – (–79) = –71. Cancel the adjacent minus signs to turn the problem into addition:

–150 – (–79) = –150 + 79

Now switch around the numbers (with their signs) to turn the problem back into subtraction: 79 – 150 = –71
i Solve the following multiplication problems:

a. 7 · 11 = 77
b. –7 · 11 = –77
c. 7 · –11 = –77
d. –7 · –11 = 77
j Solve the following division problems:

a. 32 ÷ –8 = –4
b. –32 ÷ –8 = 4
c. –32 ÷ 8 = –4
d. 32 ÷ 8 = 4
k –65 · 23 = –1,495. First, drop the signs and multiply:

65 · 23 = 1,495

The numbers –65 and 23 have different signs, so the answer is negative: –1,495.

l –143 · –77 = 11,011. Drop the signs and multiply:

143 · 77 = 11,011

The numbers –143 and –77 have the same sign, so the answer is positive: 11,011.

m 216 ÷ –9 = –24. Drop the signs and divide (use long division, as I show you in Chapter 1): 216 ÷ 9 = 24

The numbers 216 and –9 have different signs, so the answer is negative: –24.

n –3,375 ÷ –25 = 135. First, drop the signs and divide:

3,375 ÷ 25 = 135

The numbers –3,375 and –25 have the same sign, so the answer is positive: 135.

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48 Part I: Back to Basics with Basic Math

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Chapter 4

It’s Just an Expression

In This Chapter

ᮣ Evaluating Big Four expressions

ᮣ Knowing how to evaluate powers

ᮣ Putting parentheses into the mix

ᮣ Getting clear on the order of operations

An arithmetic expression is any string of numbers and operators that can be calculated. In some cases, the calculation is easy. For example, you can calculate 2 + 2 in your head to come up with the answer 4. As expressions become longer, however, the calculation becomes more difficult. You may have to spend more time with the expression 2 · 6 + 23 – 10 + 13 to find the correct answer of 38.

The word evaluate comes from the word value. When you evaluate an expression, you turn it from a string of mathematical symbols into a single value — that is, you turn it into one number. But as expressions get more complicated, the potential for confusion arises. For example, think about the expression 3 + 2 · 4. If you add the first two numbers and then multiply, your answer is 20. But if you multiply the last two numbers and then add, your answer is 11.

To solve this problem, mathematicians have agreed on an order of operations (sometimes called order of precedence): a set of rules for deciding how to evaluate an arithmetic expression no matter how complex it gets. In this chapter, I introduce you to the order of operations through a series of exercises that introduce the basic concepts one at a time. When you finish this chapter, you should be able to evaluate just about any expression your teacher can give you!

Evaluating Expressions with Addition
and Subtraction

When an expression has only addition and subtraction, in any combination, it’s easy to evaluate: Just start with the first two numbers and continue from left to right. Even when an expression includes negative numbers, the same procedure applies (just make sure you use the correct rule for adding or subtracting negative numbers, as I discuss in Chapter 3).

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50 Part I: Back to Basics with Basic Math

Q. Find 7 + –3 – 6 – (–10).

Finish up with the last two numbers,
remembering that subtracting a negative

A. 8. Start at the left with the first two num-
number is the same thing as adding a
bers, 7 + –3 = 4:

positive number:

7 + –3 – 6 – (–10) = 4 – 6 – (–10)

–2 – (–10) = –2 + 10 = 8

Continue with the next two numbers,

4 – 6 = –2:

4 – 6 – (–10) = –2 – (–10)

1. What’s 9 – 3 + 8 – 7?

2. Evaluate 11 – 5 – 2 + 6 – 12.

Solve It

Solve It

3. Find 17 – 11 – (–4) + –10 – 8.

4. –7 + –3 – (–11) + 8 – 10 + –20 = ?

Solve It

Solve It

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51

Evaluating Expressions with Multiplication
and Division

Unless you’ve been using your free time to practice your Persian, Arabic, or Hebrew reading skills, you’re probably used to reading from left to right. Luckily, that’s the direction of choice for multiplication and division problems, too. When an expression has only multiplication and division, in any combination, you should have no trouble evaluating it: Just start with the first two numbers and continue from left to right.

Q. What is 15 ÷ 5 · 8 ÷ 6?

Q. Evaluate –10 · 2 · –3 ÷ –4.

A. 4. Start at the left with the first two

A. –15. The same procedure applies when
numbers, 15 ÷ 5 = 3:

you have negative numbers (just make
sure you use the correct rule for multi-

15 ÷ 5 · 8 ÷ 6 = 3 · 8 ÷ 6
plying or dividing by negative numbers,
as I explain in Chapter 3). Start at the left

Continue with the next two numbers,
with the first two numbers, –10 · 2 = –20:

3 · 8 = 24:

–10 · 2 · –3 ÷ –4 = –20 · –3 ÷ –4

3 · 8 ÷ 6 = 24 ÷ 6

Continue with the next two numbers,

Finish up with the last two numbers:

–20 · –3 = 60:

24 ÷ 6 = 4

–20 · –3 ÷ –4 = 60 ÷ –4

Finish up with the last two numbers:

60 ÷ –4 = –15

5. Find 18 ÷ 6 · 10 ÷ 6.

6. Evaluate 20 ÷ 4 · 8 ÷ 5 ÷ –2.

Solve It

Solve It

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52 Part I: Back to Basics with Basic Math

7. 12 ÷ –3 · –9 ÷ 6 · –7 = ?

8. Solve –90 ÷ 9 · –8 ÷ –10 ÷ 4 · –15.

Solve It

Solve It

Making Sense of Mixed-Operator Expressions

Things get a little complicated in this section, but you can handle it. A mixed-operator expression contains at least one addition or subtraction sign and at least one multiplication or division sign. To evaluate mixed-operator expressions, follow a couple of simple steps: 1. Evaluate all multiplication and division from left to right.

Begin evaluating a mixed-operator expression by underlining all the multiplication or division in the problem.

2. Evaluate addition and subtraction from left to right.

Q. What’s –15 · 3 ÷ –5 – (–3) · –4?

–15 · 3 ÷ –5 – (–3) · –4

= –45 ÷ –5 – (–3) · –4

A. –3. Start by underlining all the multiplica-
tion and division in the problem; then

= 9 – (–3) · –4
evaluate all multiplication and division

= 9 – 12
from left to right:

Finish up by evaluating the addition and
subtraction from left to right:

= –3

9. Evaluate 8 – 3 · 4 ÷ 6 + 1.

10. Find 10 · 5 – (–3) · 8 ÷ –2.

Solve It

Solve It

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Chapter 4: It’s Just an Expression

53

11. –19 – 7 · 3 + –20 ÷ 4 – 8 = ?

12. What’s 60 ÷ –10 – (–2) + 11 · 8 ÷ 2?

Solve It

Solve It

Handling Powers Responsibly

You may have heard that power corrupts, but rest assured that when mathematicians deal with powers, the order of operations usually keeps them in line. When an expression contains one or more powers, evaluate all powers from left to right before moving on to the Big Four operators. Here’s the breakdown:

1. Evaluate all powers from left to right.

In Chapter 2, I show you that raising a number to a power simply means multiplying the number by itself that many times. For example, 23 = 2 · 2 · 2 = 8. Remember that anything raised to the 0 power equals 1.

2. Evaluate all multiplication and division from left to right.

3. Evaluate addition and subtraction from left to right.

If you compare this numbered list with the one in the preceding section, you’ll notice the only difference is that I’ve now inserted a new rule at the top.

Q. Evaluate 7 – 42 ÷ 24 + 9 · 23.

Next, evaluate all multiplication and divi-
sion from left to right:

A. 78. Evaluate all powers from left to right,

= 7 – 1 + 9 · 8
starting with 42 = 4 · 4 = 16:

= 7 – 1 + 72

= 7 – 16 ÷ 24 + 9 · 23

Finish up by evaluating the addition and

Move on to evaluate the remaining two
subtraction from left to right:

powers:

= 6 + 72

= 7 – 16 ÷ 16 + 9 · 8

= 78

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54 Part I: Back to Basics with Basic Math

13. Evaluate 32 – 23 ÷ 22.

14. Find 52 – 42 – (–7) · 22.

Solve It

Solve It

15. 701 – 34 ÷ –9 · –7 + 1230 = ?

16. What’s 112 – 27 + 35 ÷ 33?

Solve It

Solve It

Prioritizing Parentheses

Did you ever go to the post office and send a package high-priority so that it’d arrive as soon as possible? Parentheses work just like that. Parentheses — ( ) — allow you to indicate that a piece of an expression is high-priority — that is, it has to be evaluated before the rest of the expression.

When an expression includes parentheses with only Big Four operators, just do the following: 1. Evaluate the contents of the parentheses.

2. Evaluate Big Four operators (as I show you earlier in “Making Sense of Mixed-Operator Expressions”).

When an expression has more than one set of parentheses, don’t panic. Start by evaluating the contents of the first set and move left to right. Piece of cake!

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55

Q. Evaluate (6 – 2) + (15 ÷ 3).

Q. Evaluate (6 + 1) · (5 – (–14) ÷ –7).

A. 9. Start by evaluating the contents of the

A. 21. When a set of parentheses includes
first set of parentheses:

a mixed-operator expression, evaluate
everything inside the parentheses

(6 – 2) + (15 ÷ 3) = 4 + (15 ÷ 3)

according to the order of operations

(see “Making Sense of Mixed-Operator

Move on to the next set of parentheses:

Expressions”). Begin by evaluating the

= 4 + 5
contents of the first set of parentheses:

6 + 1 = 7:

To finish up, evaluate the addition:

(6 + 1) · (5 – (–14) ÷ –7) =

= 9

7 · (5 – (–14) ÷ –7)

Move to the next set of parentheses. This
set contains a mixed-operator expression,
so start with the division: –14 ÷ –7 = 2:

= 7 · (5 – 2)

Complete the contents of the parentheses
by evaluating the subtraction: 5 – 2 = 3:

= 7 · 3

Finish up by evaluating the multipli-
cation: 7 · 3 = 21.

17. Evaluate 4 · (3 + 4) – (16 ÷ 2).

18. What’s (5 + –8 ÷ 2) + (3 · 6)?

Solve It

Solve It

19. Find (4 + 12 ÷ 6 · 7) – (3 + 8).

20. (2 · –5) – (10 – 7) · (13 + –8) = ?

Solve It

Solve It

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56 Part I: Back to Basics with Basic Math

Pulling Apart Parentheses and Powers

When an expression has parentheses and powers, evaluate it in the following order: 1. Contents of parentheses

Writing an expression like an exponent (a small, raised number indicating a power) groups that expression like parentheses do. Evaluate any superscripted expression down to a single number before evaluating the power. In other words, to find 53 – 1, you can pretend 3 – 1 is in parentheses, making the problem 5(3 – 1) = 52 = 25.

A few other symbols that you may be familiar with also group expressions together just like parentheses. These include the square root symbol and absolute value bars, which I introduce in Chapter 2 and Chapter 3, respectively.

2. Powers from left to right

3. Multiplication and division from left to right

4. Addition and subtraction from left to right

Compare this numbered list with the one in the previous section “Handling Powers Responsibly.” The only real difference is that I’ve now inserted a new rule at the top.

Q. Evaluate (8 + 62) ÷ (23 – 4).

Q. Find the value of –1 + (–20 + 33)2.

A. 11. Begin by evaluating the contents of

A. 48. When the entire contents of a set of
the first set of parentheses. Inside this
parentheses is raised to a power, evalu-
set, evaluate the power first and do the
ate what’s inside the parentheses before
addition next:

evaluating the power. Inside this set,
evaluate the power first and the

(8 + 62) ÷ (23 – 4)

addition next:

= (8 + 36) ÷ (23 – 4)

–1 + (–20 + 33)2 = –1 + (–20 + 27)2 = –1 + 72

= 44 ÷ (23 – 4)

Next, evaluate the power 72 = 7 · 7 = 49:

Move to the next set of parentheses,

= –1 + 49
evaluating the power first and then the
subtraction:

Finish up by evaluating the addition:

= 44 ÷ (8 – 4) = 44 ÷ 4

–1 + 49 = 48.

Finish up by evaluating the division:

44 ÷ 4 = 11.

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Chapter 4: It’s Just an Expression

57

21. Find (62 – 12) ÷ (16 ÷ 23).

22. Evaluate –10 – (2 + 32 · –4).

Solve It

Solve It

23. 72 – (3 + 32 ÷ –9)5 = ?

24. What is (10 – 114 · 8)4 ÷ 4 + 5?

Solve It

Solve It

Figuring Out Nested Parentheses

Have you ever seen nested wooden dolls? (They originated in Russia, where their ultra-cool name is matryoshka. ) This curiosity appears to be a single carved piece of wood in the shape of a doll. But when you open it up, you find a smaller doll nested inside it. And when you open up the smaller doll, you find an even smaller one hidden inside that one — and so on.

Like these Russian dolls, some arithmetic expressions contain sets of nested parentheses —
one set of parentheses inside another set. To evaluate a set of nested parentheses, start by evaluating the inner set of parentheses and work your way outward.

Parentheses — ( ) — come in a number of styles, including brackets — [ ] — and braces — { }.

These different styles help you keep track of where a statement in parentheses begins and ends. No matter what they look like, to the mathematician these different styles are all parentheses, so they all get treated the same.

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58 Part I: Back to Basics with Basic Math

Q. Find the value of {3 · [10 ÷ (6 – 4)]} + 2.

Now, evaluate what’s inside the final set
of parentheses:

A. 17. Begin by evaluating what’s inside the

= 15 + 2
inner set of parentheses: 6 – 4 = 2:

{3 · [10 ÷ (6 – 4)]} + 2 = {3 · [10 ÷ 2]} + 2

Finish up by evaluating the addition:

15 + 2 = 17.

The result is an expression with one set
of parentheses inside another set, so
evaluate what’s inside the inner set:

10 ÷ 2 = 5:

= {3 · 5} + 2

25. Evaluate 7 + {[(10 – 6) · 5] + 13}.

26. Find the value of [(2 + 3) – (30 ÷ 6)] +

(–1 + 7 · 6).

Solve It

Solve It

27. –4 + {[–9 · (5 – 8)] ÷ 3} = ?

28. Evaluate {(4 – 6) · [18 ÷ (12 – 3 · 2)]} – (–5).

Solve It

Solve It

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59

Bringing It All Together: The Order of Operations

Throughout this chapter, you work with a variety of rules for deciding how to evaluate arithmetic expressions. These rules all give you a way to decide the order in which an expression gets evaluated. All together, this set of rules is called the order of operations (or sometimes, the order of precedence). Here’s the complete order of operations for arithmetic: 1. Contents of parentheses from the inside out

2. Powers from left to right

3. Multiplication and division from left to right

4. Addition and subtraction from left to right

The only difference between this list and the one in “Pulling Apart Parentheses and Powers”

is that I’ve now added a few words to the end of Step 1 to cover nested parentheses (which I discuss in the preceding section).

Q. Evaluate [(8 · 4 + 23) ÷ 10]7 – 5.

Next, evaluate what’s inside the paren-
theses and the expression that makes up

A. 16. Start by focusing on the inner set of
the exponent:

parentheses, evaluating the power, then

= 47 – 5 = 42
the multiplication, and then the addition:

[(8 · 4 + 23) ÷ 10]7 – 5

Finish by evaluating the remaining
power: 42 = 16.

= [(8 · 4 + 8) ÷ 10]7 – 5

= [(32 + 8) ÷ 10]7 – 5

= [40 ÷ 10]7 – 5

29. Evaluate 1 + [(23 – 4) + (10 ÷ 2)2].

30. (–7 · –2 + 62 ÷ 4)9 · 2 – 17 = ?

Solve It

Solve It

31. What is {62 – [12 ÷ (–13 + 14)2] · 2}2

32. Find the value of [(123 – 112)4 –

(62 ÷ 220 – 3 · 6)]2.

Solve It

Solve It

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60 Part I: Back to Basics with Basic Math

Solutions to It’s Just an Expression

The following are the answers to the practice questions presented earlier in this chapter.

a 9 – 3 + 8 – 7 = 7. Add and subtract from left to right:

9 – 3 + 8 – 7

= 6 + 8 – 7

= 14 – 7 = 7
b 11 – 5 – 2 + 6 – 12 = –2.

11 – 5 – 2 + 6 – 12

= 6 – 2 + 6 – 12

= 4 + 6 – 12

= 10 – 12 = –2
c 17 – 11 – (–4) + –10 – 8 = –8.

17 – 11 – (–4) + –10 – 8

= 6 – (–4) + –10 – 8

= 10 + –10 – 8

= 0 – 8 = –8
d –7 + –3 – (–11) + 8 – 10 + –20 = –21.

–7 + –3 – (–11) + 8 – 10 + –20

= – 10 – (–11) + 8 – 10 + –20

= 1 + 8 – 10 + –20

= 9 – 10 + –20

= –1 + –20 = –21
e 18 ÷ 6 · 10 ÷ 6 = 5. Divide and multiply from left to right:

18 ÷ 6 · 10 ÷ 6

= 3 · 10 ÷ 6

= 30 ÷ 6 = 5
f 20 ÷ 4 · 8 ÷ 5 ÷ –2 = –4.

20 ÷ 4 · 8 ÷ 5 ÷ –2

= 5 · 8 ÷ 5 ÷ –2

= 40 ÷ 5 ÷ –2

= 8 ÷ –2 = –4

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Chapter 4: It’s Just an Expression

61
g 12 ÷ –3 · –9 ÷ 6 · –7 = –42.

12 ÷ –3 · –9 ÷ 6 · –7

= –4 · –9 ÷ 6 · –7

= 36 ÷ 6 · –7

= 6 · –7 = –42
h –90 ÷ 9 · –8 ÷ –10 ÷ 4 · –15 = 30.

–90 ÷ 9 · –8 ÷ –10 ÷ 4 · –15

= –10 · –8 ÷ –10 ÷ 4 · –15

= 80 ÷ –10 ÷ 4 · –15

= –8 ÷ 4 · –15

= –2 · –15

= 30
i 8 – 3 · 4 ÷ 6 + 1 = 7. Start by underlining and evaluating all multiplication and division from left to right:

8 – 3 · 4 ÷ 6 + 1

= 8 – 12 ÷ 6 + 1

= 8 – 2 + 1

Now evaluate all addition and subtraction from left to right:

= 6 + 1 = 7
j 10 · 5 – (–3) · 8 ÷ –2 = 38. Start by underlining and evaluating all multiplication and division from left to right:

10 · 5 – (–3) · 8 ÷ –2

= 50 – (–3) · 8 ÷ –2

= 50 – (–24) ÷ –2

= 50 – 12

Now evaluate the subtraction:

= 38
k –19 – 7 · 3 + –20 ÷ 4 – 8 = –53. Start by underlining and evaluating all multiplication and division from left to right:

–19 – 7 · 3 + –20 ÷ 4 – 8

= –19 – 21 + –20 ÷ 4 – 8

= –19 – 21 + –5 – 8

Then evaluate all addition and subtraction from left to right:

= –40 + –5 – 8 = –45 – 8 = –53

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62 Part I: Back to Basics with Basic Math
l 60 ÷ –10 – (–2) + 11 · 8 ÷ 2 = 40. Start by underlining and evaluating all multiplication and division from left to right:

60 ÷ –10 – (–2) + 11 · 8 ÷ 2

= –6 – (–2) + 11 · 8 ÷ 2

= –6 – (–2) + 88 ÷ 2

= –6 – (–2) + 44

Now evaluate all addition and subtraction from left to right:

= –4 + 44 = 40
m 32 – 23 ÷ 22 = 7. First, evaluate all powers:

32 – 23 ÷ 22 = 9 – 8 ÷ 4

Next, evaluate the division:

= 9 – 2

Finally, evaluate the subtraction:

= 7
n 52 – 42 – (–7) · 22 = 37. Evaluate all powers:

52 – 42 – (–7) · 22 = 25 – 16 – (–7) · 4

Evaluate the multiplication:

= 25 – 16 – (–28)

Finally, evaluate the subtraction from left to right:

= 9 – (–28) = 37
o 701 – 34 ÷ –9 · –7 + 1230 = 8. Evaluate all powers:

701 – 34 ÷ –9 · –7 + 1230 = 70 – 81 ÷ –9 · –7 + 1

Next, evaluate the multiplication and division from left to right:

= 70 – (–9) · –7 + 1 = 70 – 63 + 1

Evaluate the addition and subtraction from left to right:

= 7 + 1 = 8
p 112 – 27 + 35 ÷ 33 = 2. First, evaluate all powers:

112 – 27 + 35 ÷ 33 = 121 – 128 + 243 ÷ 27

Evaluate the division:

= 121 – 128 + 9

Finally, evaluate the addition and subtraction from left to right:

= –7 + 9 = 2

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Chapter 4: It’s Just an Expression

63
q 4 · (3 + 4) – (16 ÷ 2) = 20. Start by evaluating what’s inside the first set of parentheses: 4 · (3 + 4) – (16 ÷ 2)

= 4 · 7 – (16 ÷ 2)

Next, evaluate the contents of the second set of parentheses:

= 4 · 7 – 8

Evaluate the multiplication and then the subtraction:

= 28 – 8 = 20
r (5 + –8 ÷ 2) + (3 · 6) = 19. Inside the first set of parentheses, evaluate the division first and then the addition:

(5 + –8 ÷ 2) + (3 · 6)

= (5 + –4) + (3 · 6)

= 1 + (3 · 6)

Next, evaluate the contents of the second set of parentheses:

= 1 + 18

Finish up by evaluating the addition: 1 + 18 = 19.

s (4 + 12 ÷ 6 · 7) – (3 + 8) = 7. Begin by focusing on the first set of parentheses, handling all multiplication and division from left to right:

(4 + 12 ÷ 6 · 7) – (3 + 8)

= (4 + 2 · 7) – (3 + 8)

= (4 + 14) – (3 + 8)

Now do the addition inside the first set of parentheses:

18 – (3 + 8)

Next, evaluate the contents of the second set of parentheses:

= 18 – 11

Finish up by evaluating the subtraction: 18 – 11 = 7.

t (2 · –5) – (10 – 7) · (13 + –8) = –25. Evaluate the first set of parentheses, then the second, and then the third:

(2 · –5) – (10 – 7) · (13 + –8)

= –10 – (10 – 7) · (13 + –8)

= –10 – 3 · (13 + –8)

= –10 – 3 · 5

Next, do multiplication and then finish up with the subtraction:

= –10 – 15 = –25

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64 Part I: Back to Basics with Basic Math
u (62 – 12) ÷ (16 ÷ 23) = 12. Focusing on the contents of the first set of parentheses, evaluate the power and then the subtraction:

(62 – 12) ÷ (16 ÷ 23)

= (36 – 12) ÷ (16 ÷ 23)

= 24 ÷ (16 ÷ 23)

Next, work inside the second set of parentheses, evaluating the power first and then the division:

= 24 ÷ (16 ÷ 8) = 24 ÷ 2

Finish by evaluating the division: 24 ÷ 2 = 12.

v –10 – (2 + 32 · –4) = 24. Focusing on the contents of the parentheses, evaluate the power first, then the multiplication, and then the addition:

–10 – (2 + 32 · –4) = –10 – (2 + 9 · –4) = –10 – (2 + –36) = –10 – (–34)

Finish by evaluating the subtraction: –10 – (–34) = 24.

w 72 – (3 + 32 ÷ –9)5 = 17. Focusing inside the parentheses, evaluate the power first, then the division, and then the addition:

72 – (3 + 32 ÷ –9)5

= 72 – (3 + 9 ÷ –9)5

= 72 – (3 + –1)5

= 72 – 25

Next, evaluate both powers in order:

= 49 – 25 = 49 – 32

To finish, evaluate the subtraction: 49 – 32 = 17.

x (10 – 114 · 8)4 ÷4 +5 = 64. Focusing inside the first set of parentheses, evaluate the power first, then the multiplication, and then the subtraction:

(10 – 114 · 8)4 ÷ 4 +5 = (10 – 1 · 8)4 ÷ 4 +5 = (10 – 8)4 ÷ 4 + 5 = 24 ÷ 4 + 5

Next, handle the expression in the exponent, evaluating the division first and then the addition: 21 + 5 = 26

To finish, evaluate the power: 26 = 64.

y 7 + {[(10 – 6) · 5] + 13} = 40. First evaluate the inner set of parentheses: 7 + {[(10 – 6) · 5] + 13} = 7 + {[4 · 5] + 13}

Move outward to the next set of parentheses:

= 7 + {20 + 13}

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65

Next, handle the remaining set of parentheses:

= 7 + 33

To finish, evaluate the addition: 7 + 33 = 40.

A [(2 + 3) – (30 ÷ 6)] + (–1 + 7 · 6) = 41. Start by focusing on the first set of parentheses. This set contains two inner sets of parentheses, so evaluate these two sets from left to right:

[(2 + 3) – (30 ÷ 6)] + (–1 + 7 · 6)

= [5 – (30 ÷ 6)] + (–1 + 7 · 6)

= [5 – 5] + (–1 + 7 · 6)

Now, the expression has two separate sets of parentheses, so evaluate the first set:

= 0 + (–1 + 7 · 6)

Handle the remaining set of parentheses, evaluating the multiplication first and then the addition:

= 0 + (–1 + 42) = 0 + 41

To finish, evaluate the addition: 0 + 41 = 41.

B –4 + {[–9 · (5 – 8)] ÷ 3} = 5. Start by evaluating the inner set of parentheses:

–4 + {[–9 · (5 – 8)] ÷ 3} = –4 + {[–9 · –3] ÷ 3}

Move outward to the next set of parentheses:

= –4 + [27 ÷ 3]

Next, handle the remaining set of parentheses:

= –4 + 9

Finally, evaluate the addition: –4 + 9 = 5.

C {(4 – 6) · [18 ÷ (12 – 3 · 2)]} – (–5) = –1. Focus on the inner set of parentheses, (12 – 3 · 2). Evaluate the multiplication first and then the subtraction:

{(4 – 6) · [18 ÷ (12 – 3 · 2)]} – (–5)

= {(4 – 6) · [18 ÷ (12 – 6)]} – (–5)

= {(4 – 6) · [18 ÷ 6]} – (–5)

Now the expression is an outer set of parentheses with two inner sets. Evaluate these two inner sets of parentheses from left to right:

= {–2 · [18 ÷ 6]} – (–5) = {–2 · 3} – (–5)

Next, evaluate the final set of parentheses:

= –6 – (–5)

Finish by evaluating the subtraction: –6 – (–5) = –1.

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D 1 + [(23 – 4) + (10 ÷ 2)2] = 30. Start by focusing on the first of the two inner sets of parentheses, (23 – 4). Evaluate the power first and then the subtraction:

1 + [(23 – 4) + (10 ÷ 2)2] = 1 + [(8 – 4) + (10 ÷ 2)2] = 1 + [4 + (10 ÷ 2)2]

Continue by focusing on the remaining inner set of parentheses:

= 1 + [4 + 52]

Next, evaluate what’s inside the last set of parentheses, evaluating the power first and then the addition:

= 1 + [4 + 25] = 1 + 29

Finish by adding the remaining numbers: 1 + 29 = 30.

E (–7 · –2 + 62 ÷ 4)9 · 2 – 17 = 23. Start with the first set of parentheses. Evaluate the power first, then the multiplication and division from left to right, and then the addition:

(–7 · –2 + 62 ÷ 4)9 · 2 – 17

= (–7 · –2 + 36 ÷ 4)9 · 2 – 17

= (14 + 36 ÷ 4)9 · 2 – 17

= (14 + 9)9 · 2 – 17

= 239 · 2 – 17

Next, work on the exponent, evaluating the multiplication first and then the subtraction:

= 2318 – 17 = 231

Finish by evaluating the power: 231 = 23.

F 62 – [12 ÷ (–13 + 14)2] · 2}2 = 144. Start by evaluating the inner set of parentheses (–13 + 14):

{62 – [12 ÷ (–13 + 14)2] · 2}2

= {62 – [12 ÷ 12] · 2}2

Move outward to the next set of parentheses, [12 ÷ 12], evaluating the power and then the division:

= {62 – [12 ÷ 1] · 2}2

= {62 – 12 · 2}2

Next, work on the remaining set of parentheses, evaluating the power, then the multiplication, and then the subtraction:

= {36 – 12 · 2}2

= {36 – 24}2

= 122

Finish by evaluating the power: 122 = 144.

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G [(123 – 112)4 – (62 ÷ 220 – 3 · 6)]2 = 49. Start by working on the exponent, 20 – 3 · 6, evaluating the multiplication and then the subtraction:

[(123 – 112)4 – (62 ÷ 220 – 3 · 6)]2

= [(123 – 112)4 – (62 ÷ 220 – 18)]2

= [(123 – 112)4 – (62 ÷ 22)]2

The result is an expression with two inner sets of parentheses. Focus on the first of these two sets, evaluating the power and then the subtraction:

= [(123 – 121)4 – (62 ÷ 22)]2

= [24 – (62 ÷ 22)]2

Work on the remaining inner set of parentheses, evaluating the two powers from left to right and then the division:

= [24 – [36÷ 22)]2

= [24 – (36÷ 4)]2

= [24 – 9]2

Now evaluate what’s left inside the parentheses, evaluating the power and then the subtraction:

= [16 – 9]2

= 72

Finish by evaluating the power: 72 = 49.

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Chapter 5

Dividing Attention: Divisibility,

Factors, and Multiples

In This Chapter

ᮣ Testing numbers for divisibility without dividing

ᮣ Understanding factors and multiples

ᮣ Distinguishing between prime and composite numbers

ᮣ Finding the factors and multiples of a number

ᮣ Finding the greatest common factor (GCF) and least common multiple (LCM)

This chapter provides an important bridge between the Big Four operations earlier in the book and the topic of fractions coming up. And right upfront, here’s the big secret: Fractions are really just division. So before you tackle fractions, the focus here is on divisibility — when you can evenly divide one number by another without getting a remainder.

First, I show you a few handy tricks for finding out whether one number is divisible by another — without actually having to divide. Next, I introduce the concepts of factors and multiples, which are closely related to divisibility.

The next few sections focus on factors. I show you how to distinguish between prime numbers (numbers that have exactly two factors) and composite numbers (numbers that have three or more factors). Next, I show you how to find all the factors of a number and how to break a number down into its prime factors. Most importantly, I show how to find the greatest common factor (GCF) of a set of numbers.

After that, I discuss multiples in detail. You discover how to generate multiples of a number, and I also give you a method for determining the least common multiple (LCM) of a set of numbers. By the end of this chapter, you should be well equipped to divide and conquer the fractions in Chapters 6 and 7.

Checking for Leftovers: Divisibility Tests

When one number is divisible by a second number, you can divide the first number by the second without having anything left over. For example, 16 is divisible by 8 because 16 ÷ 8 = 2, with no remainder. You can use a bunch of tricks for testing divisibility without actually doing the division.

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70 Part I: Back to Basics with Basic Math

The most common tests are for divisibility by 2, 3, 5, and 11. Testing for divisibility by 2 and 5
are child’s play; testing for divisibility by 3 and 11 requires a little more work. Here are some quick tests:

ߜ By 2: Any number that ends in an even number (2, 4, 6, 8, or 0) is even — that is, it’s divisible by 2. All numbers ending in an odd number (1, 3, 5, 7, or 9) are odd — that is, they aren’t divisible by 2.

ߜ By 3: Any number whose digital root is 3, 6, or 9 is divisible by 3; all other numbers (except 0) aren’t. To find the digital root of a number, just add up the digits. If the result has more than one digit, add up those digits and repeat until your result has one digit.

ߜ By 5: Any number that ends in 5 or 0 is divisible by 5; all other numbers aren’t.

ߜ By 11: Alternately place plus signs and minus signs in front of all the digits and find the answer. If the result is 0 or any number that’s divisible by 11 (even if this result is a negative number), the number is divisible by 11; otherwise, it isn’t. Remember: Always put a plus sign in front of the first number.

Q. Which of the following numbers are

Q. Which of the following numbers are
divisible by 3?

divisible by 11?

a. 31
a. 71
b. 54
b. 154
c. 768
c. 528
d. 2,809
d. 28,094

A. Add the digits to determine each

A. Place + and – signs between the numbers
number’s digital root; if the digital root is
and determine whether the result is 0 or

3, 6, or 9, the number’s divisible by 3:

a multiple of 11:

a. 31: No, because 3 + 1 = 4
a. 71: No, because +7 – 1 = 6 (check:

(check: 31 ÷ 3 = 10 r 1)

71 ÷ 11 = 6 r 5)

b. 54: Yes, because 5 + 4 = 9
b. 154: Yes, because +1 – 5 + 4 = 0

(check: 54 ÷ 3 = 18)

(check: 154 ÷ 11 = 14)

c. 768: Yes, because 7 + 6 + 8 = 21 and
c. 528: Yes, because +5 – 2 + 8 = 11

2 + 1 = 3 (check: 768 ÷ 3 = 256)

(check: 528 ÷ 11 = 48)

d. 2,809: No, because 2 + 8 + 0 + 9 = 19,
d. 28,094: Yes, because +2 – 8 + 0 – 9 + 4 =

1 + 9 = 10, and 1 + 0 = 1 (check: 2,809 ÷

–11 (check: 28,094 ÷ 11 = 2,554)

3 = 936 r 1)

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Chapter 5: Dividing Attention: Divisibility, Factors, and Multiples

71

1. Which of the following numbers are divis-

2. Which of the following numbers are divis-
ible by 2?

ible by 5?

a. 37
a. 75
b. 82
b. 103
c. 111
c. 230
d. 75,316
d. 9,995

Solve It

Solve It

3. Which of the following numbers are divis-

4. Which of the following numbers are divis-
ible by 3?

ible by 11?

a. 81
a. 42
b. 304
b. 187
c. 986
c. 726
d. 4,444,444
d. 1,969

Solve It

Solve It

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72 Part I: Back to Basics with Basic Math

Understanding Factors and Multiples

In the preceding section, I introduce the concept of divisibility. For example, 12 is divisible by 3 because 12 ÷ 3 = 4, with no remainder. You can also describe this relationship between 12
and 3 using the words factor and multiple. When you’re working with positive numbers, the factor is always the smaller number and the multiple is the bigger number. For example, 12 is divisible by 3, so

ߜ The number 3 is a factor of 12.

ߜ The number 12 is a multiple of 3.

Q. The number 40 is divisible by 5, so which

Q. Which two of the following statements
number is the factor and which is the
means the same thing as “18 is a multiple
multiple?

of 6”?

A.

a. 6 is a factor of 18.

The number 5 is the factor and 40 is the
multiple, because 5 is smaller and 40 is
b. 18 is divisible by 6.

larger.

c. 6 is divisible by 18.

d. 18 is a factor of 6.

A. Choices a and b. The number 6 is the
factor and 18 is the multiple, because 6
is smaller than 18, so a is correct. And

18 ÷ 6 = 3, so 18 is divisible by 6; there-
fore, b is correct.

5. Which of the following statements are true,

6. Which two of these statements mean the
and which are false?

same thing as “18 is divisible by 6”?

a. 5 is a factor of 15.

a. 18 is a factor of 6.

b. 9 is a multiple of 3.

b. 18 is a multiple of 6.

c. 11 is a factor of 12.

c. 6 is a factor of 18.

d. 7 is a multiple of 14.

d. 6 is a multiple of 18.

Solve It

Solve It

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Chapter 5: Dividing Attention: Divisibility, Factors, and Multiples

73

7. Which two of these statements mean the

8. Which of the following statements are true,
same thing as “10 is a factor of 50”?

and which are false?

a. 10 is divisible by 50.

a. 3 is a factor of 42.

b. 10 is a multiple of 50.

b. 11 is a multiple of 121.

c. 50 is divisible by 10.

c. 88 is a multiple of 9.

d. 50 is a multiple of 10.

d. 11 is a factor of 121.

Solve It

Solve It

One Number, Indivisible: Identifying Prime

(And Composite) Numbers

Every counting number greater than 1 is either a prime number or a composite number.

A prime number has exactly two factors — 1 and the number itself. For example, the number 5 is prime because its only two factors are 1 and 5. A composite number has at least three factors. For example, the number 4 has three factors: 1, 2, and 4. ( Note: The number 1 is the only counting number that isn’t prime or composite, because its only factor is 1.) The first six prime numbers are 2, 3, 5, 7, 11, and 13.

When testing to see whether a number is prime or composite, perform divisibility tests in the following order (from easiest to hardest): 2, 5, 3, 11, 7, and 13. If you find that a number is divisible by one of these, you know that it’s composite and you don’t have to perform the remaining tests. Here’s how you know which tests to perform:

ߜ If a number less than 121 isn’t divisible by 2, 3, 5, or 7, it’s prime; otherwise, it’s composite.

ߜ If a number less than 289 isn’t divisible by 2, 3, 5, 7, 11, or 13, it’s prime; otherwise, it’s composite.

Remember that 2 is the only prime number that’s even. The next three odd numbers are prime — 3, 5, and 7. To keep the list going, think “lucky 7, lucky 11, unlucky 13” — they’re all prime.

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74 Part I: Back to Basics with Basic Math

Q. For each of the following numbers, tell
c. 253 is composite. The number 253
which is prime and which is composite.

ends in an odd number, so it isn’t divis-
ible by 2. It doesn’t end in 5 or 0, so it
a. 185
isn’t divisible by 5. Its digital root is 1
b. 243

(because 2 + 5 + 3 = 10 and 1 + 0 = 1), so
it isn’t divisible by 3. But it is divisible
c. 253
by 11, because it passes the + and –
d. 263
test (+ 2 – 5 + 3 = 0). If you do the math,
you find that 253 = 11 · 23.

A. Check divisibility to identify prime and
d. 263 is prime. The number 263 ends in
composite numbers:

an odd number, so it isn’t divisible by 2.

a. 185 is composite. The number 185

It doesn’t end in 5 or 0, so it isn’t divis-
ends in 5, so it’s divisible by 5.

ible by 5. Its digital root is 2 (because

2 + 6 + 3 = 11 and 1 + 1 = 2), so it isn’t
b. 243 is composite. The number 243 ends
divisible by 3. It isn’t divisible by 11,
in an odd number, so it isn’t divisible by
because it fails the + and – test (+2 – 6 +

2. It doesn’t end in 5 or 0, so it isn’t divis-

3 = 1, which isn’t 0 or divisible by 11). It
ible by 5. Its digital root is 9 (because 2 +
isn’t divisible by 7, because 263 ÷ 7 = 37

4 + 3 = 9), so it’s divisible by 3. The math
r 4. And it isn’t divisible by 13, because
shows you that 243 ÷ 3 = 81.

263 ÷ 13 = 20 r 3.

9. Which of the following numbers are prime,

10. Of the following numbers, tell which are
and which are composite?

prime and which are composite.

a. 3
a. 65
b. 9
b. 73
c. 11
c. 111
d. 14
d. 172

Solve It

Solve It

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75

11. Find out whether each of these numbers is

12. Figure out which of the following are prime
prime or composite.

numbers and which are composite
numbers.

a. 23
a. 143
b. 51
b. 169
c. 91
c. 187
d. 113
d. 283

Solve It

Solve It

Generating a Number’s Factors

When one number is divisible by a second number, that second number is a factor of the first.

For example, 10 is divisible by 2, so 2 is a factor of 10.

The lowest factor of every positive number is 1, and the highest factor of every positive number is that number itself. For example, the lowest and highest factors of 12 are 1 and 12.

The rest of the factors fall somewhere in between these two numbers.

Here’s how to find all the factors of a number:

1. Write down 1 and the number itself (the lowest and highest factors), leaving some space between them.

2. Check for divisibility by 2.

If the number is divisible by 2, write down 2 as the second-lowest factor; divide the original number by 2 to get the second-highest factor.

3. Check for divisibility by 3, 4, 5, and so on until the beginning of the list meets the end.

See the earlier section titled “Checking for Leftovers: Divisibility Tests” for a few shortcuts.

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76 Part I: Back to Basics with Basic Math

Q. Write down all the factors of 18.

The digital root of 18 is 9 (because 1 + 8 =

9), so 18 is divisible by 3. And 18 ÷ 3 = 6,

A. 1, 2, 3, 6, 9, 18. The highest and lowest
so the next-highest and next-lowest fac-
factors of 18 are 1 and 18:

tors are 3 and 6:

1, . . . , 18

1, 2, 3, . . . , 6, 9, 18

The number 18 is even, so it’s divisible

The number 18 isn’t divisible by 4, because
by 2. And 18 ÷ 2 = 9, so the next-highest

18 ÷ 4 = 4 r 2. And 18 isn’t divisible by 5,
and next-lowest factors are 2 and 9:

because it doesn’t end with 5 or 0. The list
of factors is complete.

1, 2, . . . , 9, 18

13. Find all the factors of 12.

14. Write down all the factors of 28.

Solve It

Solve It

15. Figure out all the factors of 40.

16. Find all the factors of 66.

Solve It

Solve It

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77

Decomposing a Number into Its Prime Factors

Every number is the product of a unique set of prime factors, a group of prime numbers (including repeats) that, when multiplied together, equals that number. This section shows you how to find those prime factors for a given number, a process called decomposition.

An easy way to decompose a number is to make a factorization tree. Here’s how: 1. Find two numbers that multiply to equal the original number; write them as numbers that branch off the original one.

Knowing the multiplication table can often help you here.

2. If either number is prime, circle it and end that branch.

3. Continue branching off non-prime numbers into two factors; whenever a branch reaches a prime number, circle it and close the branch.

When every branch ends in a circled number, you’re finished — just gather up the circled numbers.

Q. Decompose the number 48 into its prime

Circle the prime numbers and close
factors.

those branches. At this point, the only
open branch is 4. Break it down into 2

A. 48 = 2 · 2 · 2 · 2 · 3. Begin making a fac-
and 2:

torization tree by finding two numbers

48
that multiply to equal 48:

6

8

48

2

3

2

4

6

8

2

2

Continue making branches of the tree by

Every branch ends in a circled number,
doing the same for 6 and 8:

so you’re finished. The prime factors are

2, 2, 2, 2, and 3.

48

6

8

2

3

2

4

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78 Part I: Back to Basics with Basic Math

17. Decompose 18 into its prime factors.

18. Decompose 42 into its prime factors.

Solve It

Solve It

19. Decompose 81 into its prime factors.

20. Decompose 120 into its prime factors.

Solve It

Solve It

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Finding the Greatest Common Factor (GCF)

The greatest common factor (GCF) of a set of numbers is the highest number that’s a factor of every number in that set. Finding the GCF is helpful when you want to reduce a fraction to its lowest terms (see Chapter 6).

You can find the GCF in two ways. The first option is to list all the factors of the numbers and to choose the highest one that appears in both (or all) the lists. (For info on finding factors, see the earlier section “Generating a Number’s Factors.”)

The other method uses prime factors, which I discuss in the preceding section. Here’s how to find the GCF:

1. Decompose the numbers into their prime factors.

2. Underline the factors that all the original numbers have in common.

3. Multiply the underlined numbers to get the GCF.

Q. Find the greatest common factor of 12

Q. Find the greatest common factor of 24,
and 20.

36, and 42.

A. 4. Write down all the factors of 12 and 20:

A. 6. Decompose all three numbers down to
their prime factors:

Factors of 12: 1, 2, 3, 4, 6, 12

24 = 2 · 2 · 2 · 3

Factors of 20: 1, 2, 4, 5, 10, 20

36 = 2 · 2 · 3 · 3

The number 4 is the greatest number

42 = 2 · 3 · 7
that appears on both lists, so it’s the

GCF.

Underline all factors that are common to
all three numbers:

24 = 2 · 2 · 2 · 3

36 = 2 · 2 · 3 · 2

42 = 2 · 3 · 7

Multiply those underlined numbers to
get your answer:

2 · 3 = 6

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80 Part I: Back to Basics with Basic Math

21. Find the greatest common factor of 10

22. What’s the GCF of 8 and 32?

and 22.

Solve It

Solve It

23. Find the GCF of 30 and 45.

24. Figure out the GCF of 27 and 72.

Solve It

Solve It

25. Find the GCF of 15, 20, and 35.

26. Figure out the GCF of 44, 56, and 72.

Solve It

Solve It

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81

Generating the Multiples of a Number

Generating the multiples of a number is easier than generating the factors: Just multiply the number by 1, 2, 3, and so forth. But unlike the factors of a number — which are always less than the number itself — the multiples of a number (except 0 and the number itself) are greater than that number. Therefore, you can never write down all the multiples of any number (except 0).

Q. Find the first six (positive) multiples of 4.

Q. List the first six multiples of 12.

A. 4, 8, 12, 16, 20, 24. Write down the

A. 12, 24, 36, 48, 60, 72. Write down 12 and
number 4 and keep adding 4 to it until
keep adding 12 to it until you’ve written
you’ve written down six numbers.

down six numbers.

27. Write the first six multiples of 5.

28. Generate the first six multiples of 7.

Solve It

Solve It

29. List the first ten multiples of 8.

30. Write the first six multiples of 15.

Solve It

Solve It

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82 Part I: Back to Basics with Basic Math

Finding the Least Common Multiple (LCM)

The least common multiple (LCM) of a set of numbers is the lowest number that’s a multiple of every number in that set. For small numbers, you can simply list the first several multiples of each number until you get a match.

When you’re finding the LCM of two numbers, you may want to list the multiples of the higher number first, stopping when the number of multiples you’ve written down equals the lower number. Then list the multiples of the lower number and look for a match.

However, you may have to write down a lot of multiples with this method, and the disadvantage becomes even greater when you’re trying to find the LCM of large numbers. I recom-mend a method that uses prime factors when you’re facing big numbers or more than two numbers. Here’s how:

1. Write down the prime decompositions of all the numbers.

See the earlier “Decomposing a Number into Its Prime Factors” section for details.

2. For each prime number you find, underline the most repeated occurrences of each.

In other words, compare the decompositions. If one breakdown contains two 2s and another contains three 2s, you’d underline the three 2s. If one decomposition contains one 7 and the rest don’t have any, you’d underline the 7.

3. Multiply the underlined numbers to get the LCM.

Q. Find the LCM of 6 and 8.

Q. Find the LCM of 12, 15, and 18.

A. 24. Because 8 is the higher number, write

A. 180. Begin by writing the prime decom-
down six multiples of 8:

positions of all three numbers. Then, for
each prime number you find, underline

Multiples of 8: 8, 16, 24, 32, 40, 48
the most repeated occurrences of each:

Now, write down multiples of 6 until you

12 = 2 · 2 · 3
find a matching number:

15 = 3 · 5

Multiples of 6: 6, 12, 18, 24

18 = 2 · 3 · 3

Notice that 2 appears in the decomposi-
tion of 12 most often (twice), so I under-
line both of those 2s. Similarly, 3 appears
in the decomposition of 18 most often

(twice), and 5 appears in the decomposi-
tion of 15 most often (once). Now, multi-
ply all the underlined numbers:

2 · 2 · 3 · 3 · 5 = 180

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Chapter 5: Dividing Attention: Divisibility, Factors, and Multiples

83

31. Find the LCM of 4 and 10.

32. Find the LCM of 7 and 11.

Solve It

Solve It

33. Find the LCM of 9 and 12.

34. Find the LCM of 18 and 22.

Solve It

Solve It

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84 Part I: Back to Basics with Basic Math

Solutions to Divisibility, Factors,
and Multiples

The following are the answers to the practice questions presented earlier in this chapter.

a Which of the following numbers are divisible by 2?

a. 37: No, because it’s odd (check: 37 ÷ 2 = 18 r 1)

b. 82: Yes, because it’s even (check: 82 ÷ 2 = 41)

c. 111: No, because it’s odd (check: 111 ÷ 2 = 55 r 1)

d. 75,316: Yes, because it’s even (check: 75,316 ÷ 2 = 37,658) b Which of the following numbers are divisible by 5?

a. 75: Yes, because it ends in 5 (check: 75 ÷ 5 = 25)

b. 103: No, because it ends in 3, not 0 or 5 (check: 103 ÷ 5 = 20 r 3) c. 230: Yes, because it ends in 0 (check: 230 ÷ 5 = 46)

d. 9,995: Yes, because it ends in 5 (check: 9,995 ÷ 5 = 1,999) c Which of the following numbers are divisible by 3? Note: The digital root ends in 3, 6, or 9 for numbers divisible by 3:

a. 81: Yes, because 8 + 1 = 9 (check: 81 ÷ 3 = 27)

b. 304: No, because 3 + 0 + 4 = 7 (check: 304 ÷ 3 = 101 r 1)

c. 986: No, because 9 + 8 + 6 = 23 and 2 + 3 = 5 (check: 986 ÷ 3 = 328 r 2) d. 4,444,444: No, because 4 + 4 + 4 + 4 + 4 + 4 + 4 = 28, 2 + 8 = 10, and 1 + 0 = 1

(check: 4,444,444 ÷ 3 = 1,481,481 r 1)

d Which of the following numbers are divisible by 11? Note: Answers add up to 0 or a multiple of 11 for numbers divisible by 11:

a. 42: No, because +4 – 2 = 2 (check: 42 ÷ 11 = 3 r 9)

b. 187: Yes, because +1 – 8 + 7 = 0 (check: 187 ÷ 11 = 17)

c. 726: Yes, because +7 – 2 + 6 = 11 (check: 726 ÷ 11 = 66)

d. 1,969: Yes, because +1 – 9 + 6 – 9 = –11 (check: 1,969 ÷ 11 = 179) e Which of the following statements are true, and which are false?

a. 5 is a factor of 15. True: 5 · 3 = 15.

b. 9 is a multiple of 3. True: 3 · 3 = 9.

c. 11 is a factor of 12. False: You can’t multiply 11 by any whole number to get 12.

d. 7 is a multiple of 14. False: The number 7 is a factor of 14.

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85
f Which of these statements means the same thing as “18 is divisible by 6”? Choices b and c.

You’re looking for something that says that the smaller number, 6, is a factor of the larger number (choice c) or one that says the larger number, 18, is a multiple of the smaller number (choice b).

g Which of these statements means the same thing as “10 is a factor of 50”? Choices c and d.

Factors are numbers you multiply to get larger ones, so you can say 50 is divisible by 10

(choice c). Multiples are the larger numbers, the products you get when you multiply two factors; you can say 50 is a multiple of 10 (choice d) because 10 · 5 = 50.

h Which of the following statements are true, and which are false?

a. 3 is a factor of 42. True: 3 · 14 = 42.

b. 11 is a multiple of 121. False: The number 11 is a factor of 121.

c. 88 is a multiple of 9. False: You can’t multiply 9 by any whole numbers to get 88 because 88 ÷ 9 = 9 r 7.

d. 11 is a factor of 121. True: 11 · 11 = 121.

i Which of the following numbers are prime, and which are composite?

a. 3 is prime. The only factors of 3 are 1 and 3.

b. 9 is composite. The factors of 9 are 1, 3, and 9.

c. 11 is prime. Eleven’s only factors are 1 and 11.

d. 14 is composite. As an even number, 14 is also divisible by 2 and therefore can’t be prime.

j Of the following numbers, tell which are prime and which are composite.

a. 65 is composite. Because 65 ends in 5, it’s divisible by 5.

b. 73 is prime. The number 73 isn’t even, doesn’t end in 5 or 0, and isn’t a multiple of 7.

c. 111 is composite. The digital root of 111 is 1 + 1 + 1 = 3, so it’s divisible by 3

(check: 111 ÷ 3 = 37).

d. 172 is composite. The number 172 is even, so it’s divisible by 2.

k Find out whether each of these numbers is prime or composite.

a. 23 is prime. The number 23 isn’t even, doesn’t end in 5 or 0, has a digital root of 5, and isn’t a multiple of 7.

b. 51 is composite. The digital root of 51 is 6, so it’s a multiple of 3 (check: 51 ÷ 3 = 17).

c. 91 is composite. The number 91 is a multiple of 7: 7 · 13 = 91.

d. 113 is prime. The number 113 is odd, doesn’t end in 5 or 0, and has a digital root of 5, so it’s not divisible by 2, 5, or 3. It’s also not a multiple of 7: 113 ÷ 7 = 16 r 1.

l Figure out which of the following are prime numbers and which are composite numbers: a. 143 is composite. +1 – 4 + 3 = 0, so 143 is divisible by 11.

b. 169 is composite. You can evenly divide 13 into 169 to get 13.

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86 Part I: Back to Basics with Basic Math
c. 187 is composite. +1 – 8 + 7 = 0, so 187 is a multiple of 11.

d. 283 is prime. The number 283 is odd, doesn’t end in 5 or 0, and has a digital root of 4; therefore, it’s not divisible by 2, 5, or 3. It’s not divisible by 11, because +2 – 8 + 3 = 3, which isn’t a multiple of 11. It also isn’t divisible by 7 (because 283 ÷ 7 = 40 r 3) or 13 (because 283 ÷ 13 = 21 r 10).

m The factors of 12 are 1, 2, 3, 4, 6, and 12. The highest and lowest factors are 1 and 12. And 12 is divisible by 2 (12 ÷ 2 = 6), so the next-highest and next-lowest factors are 2 and 6. Because 12 is divisible by 3 (12 ÷ 3 = 4), the next-highest and next-lowest factors are 3 and 4.

n The factors of 28 are 1, 2, 4, 7, 14, and 28. The highest and lowest factors are 1 and 28. And 28
is divisible by 2 (28 ÷ 2 = 14), so the next-highest and next-lowest factors are 2 and 14. Although 28 isn’t divisible by 3, it’s divisible by 4 (28 ÷ 4 = 7), so the next-highest and next-lowest factors are 4 and 7. Finally, 28 isn’t divisible by 5 or 6.

o The factors of 40 are 1, 2, 4, 5, 8, 10, 20, and 40. The highest and lowest factors are 1 and 40.

Because 40 is divisible by 2 (40 ÷ 2 = 20), the next-highest and next-lowest factors are 2 and 20.

Although 40 isn’t divisible by 3, it’s divisible by 4 (40 ÷ 4 = 10), so the next-highest and next-lowest factors are 4 and 10. And 40 is divisible by 5 (40 ÷ 5 = 8), so the next-highest and next-lowest factors are 5 and 8. Finally, 40 isn’t divisible by 6 or 7.

p The factors of 66 are 1, 2, 3, 6, 11, 22, 33, and 66. The highest and lowest factors are 1 and 66.

The number 66 is divisible by 2 (66 ÷ 2 = 33), so the next-highest and next-lowest factors are 2
and 33. It’s also divisible by 3 (66 ÷ 3 = 22), so the next-highest and next-lowest factors are 3 and 22. Although 66 isn’t divisible by 4 or 5, it’s divisible by 6 (66 ÷ 6 = 11), so the next-highest and next-lowest factors are 6 and 11. Finally, 66 isn’t divisible by 7, 8, 9, or 10.

q Decompose 18 into its prime factors. 18 = 2 · 2 · 3. Here’s one possible factoring tree: 18

3

6

2

3
r Decompose 42 into its prime factors. 42 = 2 · 3 · 7. Here’s one possible factoring tree: 42

6

7

2

3
s Decompose 81 into its prime factors. 81 = 3 · 3 · 3 · 3. Here’s one possible factoring tree: 81

9

9

3

3

3

3
t Decompose 120 into its prime factors. 120 = 2 · 2 · 2· 3 · 5. Here’s one possible factoring tree: 120

10

12

2

5

3

4

2

2

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87
u The GCF of 10 and 22 is 2. Write down all the factors of 10 and 22: 10: 1, 2, 5, 10

22: 1, 2, 11, 22

The number 2 is the greatest number that appears on both lists.

v The GCF of 8 and 32 is 8. Write down all the factors of 8 and 32:

8: 1, 2, 4, 8

32: 1, 2, 4, 8, 16, 32

The greatest number that appears on both lists is 8.

w The GCF of 30 and 45 is 15. Write down all the factors of 30 and 45: 30: 1, 2, 3, 5, 6, 10, 15, 30

45: 1, 3, 5, 9, 15, 45

The greatest number that appears on both lists is 5.

x The GCF of 27 and 72 is 9. Decompose 27 and 72 into their prime factors and underline every factor that’s common to both:

27 = 3 · 3 · 3

72 = 2 · 2 · 2 · 3 · 3

Multiply those underlined numbers to get your answer: 3 · 3 = 9.

y The GCF of 15, 20, and 35 is 5. Decompose the three numbers into their prime factors and underline every factor that’s common to all three:

15 = 3 · 5

20 = 2 · 2 · 5

35 = 5 · 7

The only factor common to all three numbers is 5.

A The GCF of 44, 56, and 72 is 4. Decompose all three numbers to their prime factors and underline each factor that’s common to all three:

44 = 2 · 2 · 11

56 = 2 · 2 · 2 · 7

72 = 2 · 2 · 2 · 3 · 3

Multiply those underlined numbers to get your answer: 2 · 2 = 4.

B The first six multiples of 5 are 5, 10, 15, 20, 25, and 30. Write down the number 5 and keep adding 5 to it until you’ve written six numbers.

C The first six multiples of 7 are 7, 14, 21, 28, 35, and 42.

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88 Part I: Back to Basics with Basic Math

D The first ten multiples of 8 are 8, 16, 24, 32, 40, 48, 56, 64, 72, and 80.

E The first six multiples of 15 are 15, 30, 45, 60, 75, and 90.

F The LCM of 4 and 10 is 20. Write down four multiples of 10:

Multiples of 10: 10, 20, 30, 40

Next, generate multiples of 4 until you find a matching number:

Multiples of 4: 4, 8, 12, 16, 20

G The LCM of 7 and 11 is 77. Write down seven multiples of 11:

Multiples of 11: 11, 22, 33, 44, 55, 66, 77

Next, generate multiples of 7 until you find a matching number:

Multiples of 7: 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77

H The LCM of 9 and 12 is 36. Write down nine multiples of 12:

Multiples of 12: 12, 24, 36, 48, 60, 72, 84, 96, 108

Next, generate multiples of 9 until you find a matching number:

Multiples of 9: 9, 18, 27, 36

I The LCM of 18 and 22 is 198. First, decompose both numbers down to their prime factors. Then underline the most frequent occurences of each prime number:

18 = 2 · 3 · 3

22 = 2 · 11

The factor 2 appears only once in any decomposition, so I underline a 2. The number 3 appears twice in the decomposition of 18, so I underline both of these. The number 11 appears only once, in the decomposition of 22, so I underline it. Now, multiply all the underlined numbers: 2 · 3 · 3 · 11 = 198

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Part II

Slicing Things Up:

Fractions, Decimals,
and Percents

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In this part . . .

This part shows you the basics of using fractions, deci-
mals, and percents so you feel confident working with
all three of these ways to slice up whole numbers. I give
you plenty of practice applying the Big Four operations

(adding, subtracting, multiplying, and dividing) to these
forms. You also discover how to convert numbers from
one form to another.

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Chapter 6

Fractions Are a Piece of Cake

In This Chapter

ᮣ Knowing the basics of fractions

ᮣ Converting between improper fractions and mixed numbers

ᮣ Increasing and reducing the terms of fractions

ᮣ Comparing fractions using cross-multiplication

Fractions come into play when you divide a whole object into equal pieces. For example, if you cut a cake into four equal pieces, each piece is 1⁄4 of the whole cake. Fractions are commonly used in everything from cooking to carpentry. You see a lot of them in math classes, too!

This chapter starts off with the basics of fractions, showing you how fractions use two numbers — the numerator (top number) and denominator (bottom number) — to represent part of a whole object. You discover how to recognize the three basic types of fractions (proper fractions, improper fractions, and mixed numbers) and how to convert back and forth between improper fractions and mixed numbers. Then I get you started increasing and reducing the terms of fractions. Finally, I show you how to cross-multiply a pair of fractions to change them to two new fractions with a common denominator. You use this trick to find out which fraction is greater and which is lesser.

Getting Down the Basic Fraction Stuff

Fractions represent parts of a whole — that is, quantities less than 1. Probably the most commonly used fraction is 1⁄2, which is one-half. When you cut a cake into two pieces and take one for yourself, you get 1⁄2 of the cake — I hope you’re hungry! In Figure 6-1, your piece is shaded.

When you slice yourself a fraction of a cake, that fraction contains two numbers, and each number tells you something different:

ߜ The top number — called the numerator — tells you the number of shaded slices.

ߜ The bottom number — called the denominator — tells you the total number of slices.

When the numerator of a fraction is less than the denominator, that fraction is a proper fraction.

If the numerator is greater than the denominator, that fraction is an improper fraction. You can convert improper fractions into mixed numbers, as I show you in the next section.

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92 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Figure 6-1:

Half of a
cake.

Some fractions can be easily written as whole numbers:

ߜ When a fraction’s denominator is 1, that fraction is equal to its numerator.

ߜ When a fraction’s numerator and denominator are the same, that fraction is equal to 1.

(This idea is important when you want to change the terms of a fraction — see

“Increasing and Reducing the Terms of Fractions” for details.)

When you reverse the order of the numerator and denominator in a fraction, the result is the reciprocal of that fraction. You use reciprocals to divide by fractions; check out Chapter 7 for more info.

Q. For each cake pictured below, identify the fraction of the cake that’s shaded.

a
b
c
d

A. Put the number of shaded slices over the number of total slices in each cake: a. 2⁄3
b. 1⁄4
c. 5⁄8
d. 7⁄10

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Chapter 6: Fractions Are a Piece of Cake

93

Q. What’s the reciprocal of each of the

A. To find the reciprocal, switch around the
following fractions?

numerator and the denominator:

a. 3⁄4
a. The reciprocal of 3⁄4 is 4⁄3.

b. 6⁄11
b. The reciprocal of 6⁄11 is 11⁄6.

c. 22⁄7
c. The reciprocal of 22⁄7 is 7⁄22.

d. 41⁄48
d. The reciprocal of 41⁄48 is 48⁄41.

1. For each cake pictured, identify the fraction of the cake that’s shaded.

a
b
c
d

Solve It

2. Which of the following fractions are

3. Rewrite each of the following fractions as a
proper? Which are improper?

whole number:

a. 3⁄2
a. 3⁄3
b. 8⁄9
b. 10⁄1
c. 20⁄23
c. 10⁄10
d. 75⁄51
d. 81⁄1

Solve It

Solve It

4. Find the reciprocal of the following fractions:

a. 5⁄7
b. 10⁄3
c. 12⁄17
d. 80⁄91

Solve It

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94 Part II: Slicing Things Up: Fractions, Decimals, and Percents

In Mixed Company: Converting between

Mixed Numbers and Improper Fractions

When the numerator (top number) is greater than the denominator (bottom number), that fraction is an improper fraction. An alternative form for an improper fraction is as a mixed number, which is made up of a whole number and a fraction.

For example, you can represent the improper fraction 3⁄2 as the equivalent mixed number 11⁄2.

The mixed number 11⁄2 means 1 + 1⁄2. To see why 3⁄2 = 11⁄2, realize that three halves of a cake is the same as one whole cake plus another half. Every improper fraction has an equivalent mixed number, and vice versa.

Sometimes at the beginning of a fraction problem, converting a mixed number to an improper fraction makes the problem easier to solve. Here’s how to make the switch from mixed number to improper fraction:

1. Multiply the whole number by the fraction’s denominator (bottom number).

2. Add the numerator (top number) to the product from Step 1.

3. Place the sum from Step 2 over the original denominator.

Similarly, at the end of some problems, you may need to convert an improper fraction to a mixed number. To do so, simply divide the numerator by the denominator. Then build a mixed number:

ߜ The quotient is the whole number.

ߜ The remainder is the numerator of the fraction.

ߜ The denominator of the fraction stays the same.

Think of the fraction bar as a division sign.

Q. Convert the mixed number 23⁄

Q.

4 to an

Convert the mixed number 35⁄7 to an
improper fraction.

improper fraction:

A. 11⁄

A. 26

4. Multiply the whole number (2) by

⁄7. Multiply the whole number (3) by
the denominator (4), and then add the
the denominator (7), and then add the
numerator (3):

numerator (5). This time, I do the whole
process in one step:

2 · 4 + 3 = 11

(3⋅7+5) 26

Use this number as the numerator

3 5 =

=

7

7

7
of your answer, keeping the same
denominator:

11⁄4

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Chapter 6: Fractions Are a Piece of Cake

95

Q. Convert the improper fraction 11⁄

Q.

2 to a

Convert the improper fraction 39⁄5 to a
mixed number:

mixed number:

A. 51⁄

A.

2. Divide the numerator (11) by the

74⁄5. Divide the numerator (39) by the
denominator (2):

denominator (5):

Quotient

→

5

Quotient →

7

2)11

5)39

−10

−35

Remainder →

1

Remainder →

4

Build your answer using the quotient (7)

Now build a mixed number using the
as the whole number and the remainder
quotient (5) as the whole number and

(4) as the numerator, keeping the same
the remainder (1) as the numerator,
denominator (5):

keeping the same denominator (2):

74⁄5

51⁄2

5. Convert the mixed number 51⁄

6.

4 to an

Change 72⁄9 to an improper fraction.

improper fraction.

Solve It

Solve It

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96 Part II: Slicing Things Up: Fractions, Decimals, and Percents

7. Express the mixed number 105⁄

8.

12 as an

Convert the improper fraction 13⁄4 to a
improper fraction.

mixed number.

Solve It

Solve It

9. Express the improper fraction 29⁄

10.

10 as a

Change 100⁄7 to a mixed number.

mixed number.

Solve It

Solve It

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Chapter 6: Fractions Are a Piece of Cake

97

Increasing and Reducing the Terms of Fractions

When you cut a cake into two pieces and take one piece, you have 1⁄2 of the cake. And when you cut it into four pieces and take two, you have 2⁄4 of the cake. Finally, when you cut it into six pieces and take three, you have 3⁄6 of the cake. Notice that in all these cases, you get the same amount of cake. This shows you that the fractions 1⁄2, 2⁄4, and 3⁄6 are equal; so are the fractions 10⁄20 and 1,000,000⁄2,000,000.

Most of the time, writing this fraction as 1⁄2 is preferred because the numerator and denominator are the smallest possible numbers. In other words, the fraction 1⁄2 is written in lowest terms. At the end of a problem, you often need to reduce a fraction, or write it in lowest terms. There are two ways to do this — the informal way and the formal way:

ߜ The informal way to reduce a fraction is to divide both the numerator and the denominator by the same number.

Advantage: This informal way is easy.

Disadvantage: It doesn’t always reduce the fraction to lowest terms (though you do get the fraction in lowest terms if you divide by the greatest common factor, which I discuss in Chapter 5).

ߜ The formal way is to decompose both the numerator and the denominator into their prime factors and then cancel common factors.

Advantage: The formal way always reduces the fraction to lowest terms.

Disdvantage: It takes longer than the informal way.

Start every problem using the informal way. If the going gets rough and you’re still not sure whether your answer is reduced to lowest terms, switch over to the formal way.

Sometimes at the beginning of a fraction problem, you need to increase the terms of a fraction — that is, write that fraction using a greater numerator and denominator. To increase terms, multiply both the numerator and denominator by the same number.

Q. Increase the terms of the fraction 4⁄

Multiply this result by the numerator:

5 to a
new fraction whose denominator is 15:

3 · 4 = 12

A. 12⁄15. To start out, write the problem as

Finally, take this number and use it to
follows:

replace the question mark:

4 = ?

4

12

=

5

15

5

15

The question mark stands for the nu-
merator of the new fraction, which
you want to fill in. Now divide the
larger denominator (15) by the smaller
denominator (5).

15 ÷ 5 = 3

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98 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Q. Reduce the fraction 18⁄

Q.

42 to lowest terms.

Reduce the fraction 135⁄196 to lowest terms.

A. 3⁄

A. 135

7. The numerator and denominator

⁄196. The numerator and denominator
aren’t too large, so use the informal way:

are both over 100, so use the formal way.

To start out, try to find a small number

First, decompose both the numerator
that the numerator and denominator are
and denominator down to their prime
both divisible by. In this case, notice that
factors:

the numerator and denominator are both
divisible by 2, so divide both by 2:

135

(3⋅3⋅3⋅5)

=

196

(2⋅2⋅7⋅7)

18

18

( ÷2) 9

=

42

(42÷2

21

) =

The numerator and denominator have no
common factors, so the fraction is

Next, notice that the numerator and
already in lowest terms.

denominator are both divisible by 3

(see Chapter 5 for more on how to tell
whether a number is divisible by 3),
so divide both by 3:

9

(9÷3) 3

=

21

(21÷3 7

) =

At this point, there’s no number (except
for 1) that evenly divides both the
numerator and denominator, so this is
your answer.

11. Increase the terms of the fraction 2⁄

12.

3 so that

Increase the terms of 4⁄9, changing the
the denominator is 18.

denominator to 54.

Solve It

Solve It

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Chapter 6: Fractions Are a Piece of Cake

99

13. Reduce the fraction 12⁄

14.

60 to lowest terms.

Reduce 45⁄75 to lowest terms.

Solve It

Solve It

15. Reduce the fraction 135⁄

16.

180 to lowest terms.

Reduce 108⁄217 to lowest terms.

Solve It

Solve It

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100 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Comparing Fractions with Cross-Multiplication

Cross-multiplication is a handy tool for getting a common denominator for two fractions, which is important for many operations involving fractions. In this section, I show you how to cross-multiply to compare a pair of fractions to find out which is greater or less. (In Chapter 7, I show you how to use cross-multiplication to add fractions, and in Chapter 15, I show you how to use it to help solve algebra equations.)

Here’s how to cross-multiply two fractions:

1. Multiply the numerator (top number) of the first fraction by the denominator (bottom number) of the second, writing the answer below the first fraction.

2. Multiply the numerator of the second fraction by the denominator of the first, writing the answer below the second fraction.

The result is that each fraction now has a new number written underneath it. The larger number is below the larger fraction.

You can use cross-multiplication to rewrite a pair of fractions as two new fractions with a common denominator:

1. Cross-multiply the two fractions to find the numerators of the new fractions.

2. Multiply the denominators of the two fractions to find the new denominators.

When two fractions have the same denominator, then the one with the greater numerator is the greater fraction.

Q. Which fraction is greater: 5⁄

Q.

8 or 6⁄11?

Which of these three fractions is the
least: 3⁄4, 7⁄10, or 8⁄11?

A. 5⁄8 is greater. Cross-multiply the two
fractions:

A. 7⁄10 is least. Use cross-multiplication to
change the first two fractions to a new

5

6
pair of fractions with a common denomi-

8

11
nator. The new numerators are 3 · 10 = 30

55

48
and 7 · 4 = 28, and the new denominators

Multiply the denominators (8 · 11 = 88).

are 4 · 10 = 40:

Use this number as your common
denominator:

3

7

4

10

55
versus

48

↓

↓

88

88

30

28

Because 55⁄88 is greater than 48⁄88, 5⁄8 is

40

40
greater than 6⁄11.

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Chapter 6: Fractions Are a Piece of Cake 101

Because 28⁄40 is less than 30⁄40, 7⁄10 is less than

Because 77⁄110 is less than 80⁄110, 7⁄10 is less

3⁄4, so you can rule out 3⁄4. Now compare 7⁄10
than 8⁄11. Therefore, 7⁄10 is the least of the
and 8⁄11 similarly. The new numerators are
three fractions.

7 · 11 = 77 and 8 · 10 = 80, and the new
denominators are 10 · 11 = 110:

7

8

10

11

↓

↓

77

80

110

110

17. Which is the greater fraction: 1⁄

18.

5 or 2⁄9?

Find the lesser fraction: 3⁄7 or 5⁄12.

Solve It

Solve It

19. Among these three fractions, which is

20. Figure out which of the following fractions
greatest: 1⁄10, 2⁄21, or 3⁄29?

is the least: 1⁄3, 2⁄7, 4⁄13, or 8⁄25.

Solve It

Solve It

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102 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Solutions to Fractions Are a Piece of Cake

The following are the answers to the practice questions presented in this chapter.

a For each cake pictured, identify the fraction of the cake that’s shaded.

a. You have 1 shaded slice and 3 slices in total, so it’s 1⁄3.

b. You have 3 shaded slices and 4 slices in total, so it’s 3⁄4.

c. You have 5 shaded slices and 6 slices in total, so it’s 5⁄6.

d. You have 7 shaded slices and 12 slices in total, so it’s 7⁄12.

b Which of the following fractions are proper? Which are improper?

a. The numerator (3) is greater than the denominator (2), so 3⁄2 is an improper fraction.

b. The numerator (8) is less than the denominator (9), so 8⁄9 is a proper fraction.

c. The numerator (20) is less than the denominator (23), so 20⁄23 is a proper fraction.

d. The numerator (75) is greater than the denominator (51), so 75⁄51 is an improper fraction.

c Rewrite each of the following fractions as a whole number.

a. The numerator and denominator are the same, so 3⁄3 = 1.

b. The denominator is 1, so 10⁄1 = 10.

c. The numerator and denominator are the same, so 10⁄10 = 1.

d. The denominator is 1, so 81⁄1 = 81.

d Find the reciprocal of the following fractions by switching the numerator and denominator.

a. The reciprocal of 5⁄7 is 7⁄5.

b. The reciprocal of 10⁄3 is 3⁄10.

c. The reciprocal of 12⁄17 is 17⁄12.

d. The reciprocal of 80⁄91 is 91⁄80.

(5⋅4+1)

e 5 1

21

=

=

4

4

4

(7⋅9+2)

f 7 2

65

=

=

9

9

9

10

( ⋅12+5)

g 10 5

125

=

=

12

12

12

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Chapter 6: Fractions Are a Piece of Cake 103
h 13⁄4 = 31⁄4. Divide the numerator (13) by the denominator (4): Quotient →

3

4)13

−12

Remainder →

1

Build your answer using the quotient (3) as the whole number and the remainder (1) as the numerator, keeping the same denominator (4): 31⁄4.

i 29⁄10 = 29⁄10. Divide the numerator (29) by the denominator (10): Quotient →

2

10)29

−20

Remainder →

9

Build your answer using the quotient (2) as the whole number and the remainder (9) as the numerator, keeping the same denominator (10): 29⁄10.

j 100⁄7 = 142⁄7. Divide the numerator (100) by the denominator (7): Quotient →

14

7)100

−7

30

−28

Remainder →

2

Build your answer using the quotient (14) as the whole number and the remainder (2) as the numerator, keeping the same denominator (7): 142⁄7.

k 2 12

=

. To start out, write the problem as follows:

3

18

2 = ?

3

18

Divide the larger denominator (18) by the smaller denominator (3) and then multiply this result by the numerator (2):

18 ÷ 3 = 6

6 · 2 = 12

Take this number and use it to replace the question mark; your answer is 12⁄18.

l 4 24

=

. Write the problem as follows:

9

54

4 = ?

9

54

Divide the larger denominator (54) by the smaller denominator (9) and then multiply this result by the numerator (4):

54 ÷ 9 = 6

6 · 4 = 24

Take this number and use it to replace the question mark; your answer is 24⁄54.

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104 Part II: Slicing Things Up: Fractions, Decimals, and Percents
m 12 1

= . The numerator (12) and the denominator (60) are both even, so divide both by 2: 60

5

12

6

=

60

30

They’re still both even, so divide both by 2 again:

= 3

15

Now the numerator and denominator are both divisible by 3, so divide both by 3:

= 15
n 45 3

= . The numerator (45) and the denominator (75) are both divisible by 5, so divide both by 5: 75

5

45

9

=

75

15

Now the numerator and denominator are both divisible by 3, so divide both by 3:

= 35
o 135 3

= . The numerator (135) and the denominator (180) are both divisible by 5, so divide 180

4
both by 5:

135

27

=

180

36

Now the numerator and denominator are both divisible by 3, so divide both by 3:

= 9

12

They’re still both divisible by 3, so divide both by 3 again:

= 34
p 108 108

=

. With a numerator and denominator this large, reduce using the formal way. First, 217

217
decompose both the numerator and denominator down to their prime factors:

108

(2⋅2⋅3⋅3⋅3)

=

217

(7⋅31)

The numerator and denominator have no common factors, so the fraction is already in lowest terms.

q 2⁄9 is greater than 1⁄5. Cross-multiply the two fractions:

1

2

5

9

9

10

Now multiply the denominators (5 · 9 = 45) to find the common denominator:

9
versus

10

45

45

Because 10 is greater than 9 , 2 is greater than 1 .

45

45

9

5

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Chapter 6: Fractions Are a Piece of Cake 105
r 5 is less than 3 . Cross-multiply the two fractions:

12

7

3

5

7

12

36

35

Now multiply the denominators (7 · 12 = 84) to find the common denominator:

36

35

84

84

Because 35 is less than 36 , 5 is less than 3 .

84

84

12

7
s 3 is greater than 1 and 2 . Use cross-multiplication to compare the first two fractions.

29

10

21

The new numerators are 1 · 21 = 21 and 2 · 10 = 20, and the new denominators are 10 · 21 = 210.

Because 21 is greater than 20 , 1 is greater than 2 , so you can rule out 2 . Next, 210

210

10

21

21
compare 1 and 3 by cross-multiplying. The new numerators are 1 · 29 = 29 and 3 · 10 = 30, 10

29
and the new denominators are 10 · 29 = 290:

1

3

10

29

↓

↓

29

30

290

290

Because 30 is greater than 29 , 3 is greater than 1 . Therefore, 3 is the greatest of the 290

290

29

10

29
three fractions.

t 2 is less than 1 , ,

4
and

8 . Cross-multiply and find a common denominator to compare

7

3

13

25
the first two fractions. The new numerators are 1 · 7 = 7 and 2 · 3 = 6, and the new denominators are 3 · 7 = 21:

1

2

3

7

↓

↓

7

6

21

21

Because 6 is less than 7 , 2 is less than 1 , so you can rule out 1 . Next, compare 2 and 21

21

7

3

3

7

4 by cross-multiplying and finding a common denominator. The new numerators are 2 · 13 =

13

26 and 4 · 7 = 28, and the new denominators are 7 · 13 = 91:

2

4

7

13

↓

↓

26

28

91

91

Because 26 is less than 28 , 2 is less than 4 , so you can rule out 4 .

91

91

7

13

13

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106 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Finally, compare 2⁄7 and 8⁄25 by cross-multiplying and finding a common denominator. The new numerators are 2 · 25 = 50 and 8 · 7 = 56, and the new denominators are 7 · 25 = 175: 2

8

7

25

↓

↓

50

56

175

175

Because 50⁄175 is less than 56⁄175, 2⁄7 is less than 8⁄25. Therefore, 2⁄7 is the lowest of the four fractions.

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Chapter 7

Fractions and the Big Four

In This Chapter

ᮣ Multiplying and dividing fractions

ᮣ Knowing a variety of methods for adding and subtracting fractions

ᮣ Applying the Big Four operations to mixed numbers

After you get the basics of fractions (which I cover in Chapter 6), you need to know how to apply the Big Four operations — adding, subtracting, multiplying, and dividing — to fractions and mixed numbers. In this chapter, I get you up to speed. First, I show you how to multiply and divide fractions — surprisingly, these two operations are the easiest to do.

Next, I show you how to add fractions that have a common denominator (that is, fractions that have the same bottom number). After that, you discover a couple of ways to add fractions that have different denominators. Then I repeat this process for subtracting fractions.

After that, I focus on mixed numbers. Again, I start with multiplication and division and then move on to the more difficult operations of addition and subtraction. At the end of this chapter, you should have a solid understanding of how to apply each of the Big Four operations to both fractions and mixed numbers.

Multiplying Fractions: A Straight Shot

Why can’t everything in life be as easy as multiplying fractions? To multiply two fractions, just do the following:

ߜ Multiply the two numerators (top numbers) to get the numerator of the answer.

ߜ Multiply the two denominators (bottom numbers) to get the denominator of the answer.

When you multiply two proper fractions, the answer is always a proper fraction, so you won’t have to change it to a mixed number, but you may have to reduce it. (See Chapter 6
for more on reducing fractions.)

Before you multiply, see whether you can cancel out common factors that appear in both the numerator and denominator. (This process is similar to reducing a fraction.) When you cancel out all common factors before you multiply, you get an answer that’s already reduced to lowest terms.

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108 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Q.

4 5

Multiply 2⁄

Q.

⋅

5 by 4⁄9.

Find .

7 8

A. 8⁄

A. 5

45. Multiply the two numerators (top

⁄14. Before you multiply, notice that the
numbers) to get the numerator of the
numerator 4 and the denominator 8 are
answer. Then multiply the two denomina-
both even. So divide both of these num-
tors (bottom numbers) to get the de-
bers by 2 just as you would when reduc-
nominator of the answer:

ing a fraction:

( ⋅ )

4 5

2 5

2 4

2 4

8

⋅ = ⋅

⋅ =

7 8

7 4

5 9

(5⋅9 45

) =

Now the numerator 2 and the denomina-

In this case, you don’t have to reduce the
tor 4 are both even, so repeat this process:

answer.

= 1 ⋅ 5

7 2

At this point, neither numerator has a
common factor with either denominator,
so you’re ready to multiply. Multiply the
two numerators to get the numerator
of the answer. Then multiply the two
denominators to get the denominator
of the answer:

(1⋅5

5

) = 14

(7⋅2)

Because you canceled all common
factors before multiplying, this answer
is in lowest terms.

1. Multiply 2⁄

2.

3 by 7⁄9.

Find 3⁄8 · 6⁄11.

Solve It

Solve It

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Chapter 7: Fractions and the Big Four 109

3. Multiply 2⁄

4.

9 by 3⁄10.

Figure out 9⁄14 · 8⁄15.

Solve It

Solve It

Flipping for Fraction Division

Mathematicians didn’t want to muck up fraction division by making you do something as complicated as actually dividing, so they devised a way to use multiplication instead. To divide one fraction by another fraction, change the problem to multiplication: 1. Change the division sign to a multiplication sign.

2. Change the second fraction to its reciprocal.

Switch around the numerator (top number) and denominator (bottom number).

3. Solve the problem using fraction multiplication.

When dividing fractions, you may have to reduce your answer or change it from an improper fraction to a mixed number, as I show you in Chapter 6.

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110 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Q.

5

3

7

2

Divide by .

Q.

÷

8

7

Solve .

10

5

A. 111⁄

A.

24. Change the division to multiplication:

1 3⁄4. Change the division to multiplication:

7

2

7

5

5

3

5 7

÷ = ⋅

÷ =

⋅

10

5

10 2

8

7

8 3

Solve the problem using fraction

Notice that you have a 5 in one of the
multiplication:

numerators and a 10 in the other frac-
tion’s denominator, so you can cancel

(5⋅7)

out the common factor, which is 5; that

=

35

(
would change your problem to 7⁄2 · 1⁄2. Or

8 ⋅ 3) = 24
you can simply do your calculations and

The answer is an improper fraction
reduce the fraction later, as I do here.

(because the numerator is greater than

Solve by multiplying these two fractions:

the denominator), so change it to a

(7⋅5)

mixed number. Divide the numerator by

=

35

(10 2) =
the denominator and put the remainder

⋅

20
over the denominator:

This time, the numerator and denomina-
tor are both divisible by 5, so you can

= 111

24
reduce them:

= 74

Because the numerator is greater than
the denominator, the fraction is improper,
so change it to a mixed number:

= 134

5. Divide 1⁄

6.

4 by 6⁄7.

Find 3⁄5 ÷ 9⁄10.

Solve It

Solve It

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Chapter 7: Fractions and the Big Four 111

7. Divide 8⁄

8.

9 by 3⁄10.

Solve 14⁄15 ÷ 7⁄12.

Solve It

Solve It

Reaching the Common Denominator:

Adding Fractions

In this section, I show you the easy stuff first, but then things get a little trickier. Adding fractions that have the same denominator (also called a common denominator) is super easy: Just add the numerators and keep the denominator the same. Sometimes you may have to reduce the answer to lowest terms or change it from an improper fraction to a mixed number.

Adding fractions that have different denominators takes a bit of work. Essentially, you need to increase the terms of one or both fractions so the denominators match before you can add. The easiest way to do this is by using a cross-multiplication trick that switches the terms of the fractions for you. Here’s how it works:

1. Cross-multiply the two fractions (as I show you in Chapter 6).

Multiply the numerator of the first fraction by the denominator of the second fraction and multiply the numerator of the second fraction by the denominator of the first.

2. Build two fractions that have a common denominator.

Multiply the denominators of your two original fractions to get the new, common denominator. Create two new fractions by putting your results from Step 1 over this new denominator.

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112 Part II: Slicing Things Up: Fractions, Decimals, and Percents

3. Add the fractions from Step 2.

Add the numerators and leave the denominator the same.

When one denominator is a multiple of the other, you can use a quick trick to find a common denominator: Increase only the terms of the fraction with the lower denominator to make both denominators the same.

Q. Add 2⁄

Q.

7 and 3⁄7.

Add 3⁄5 and 14⁄15.

A. 5⁄

A.

7. The denominators are both 7, so add

1 8⁄15. The denominators are different, but
the numerators (2 and 3) to get the new
because 15 is a multiple of 5, you can use
numerator and keep the denominator
the quick trick described earlier. Increase
the same:

the terms of 3⁄5 so that its denominator is

(

15. To do this, you need to multiply the

+ )

2

3

2

3

5
numerator and the denominator by the

+ =

=

7

7

7

7
same number. You have to multiply 5
times 3 to get 15 in the denominator, so

Q. Find 5⁄8 + 7⁄8.

you want to multiply the numerator by 3
as well:

A. 11⁄2. The denominators are both 8, so add
the numerators (5 and 7) to get the new

( ⋅ )

3

3 3

9

=
numerator and keep the denominator the

5

(5⋅3 15

) =
same:

(

Now both fractions have the same

+ )

5

7

5 7

12
denominator, so add their numerators:

+ =

=

8

8

8

8

9

14

(9+14) 23

The numerator is greater than the de-

=

+

=

=

15

15

15

15
nominator, so the answer is an improper
fraction. Change it to a mixed number

The result is an improper fraction, so

(as I show you in Chapter 6):

change this to a mixed number:

=

=

1 4

1 8

8

15

The fractional part of this mixed number
still needs to be reduced, because the
numerator and denominator are both
even numbers. Divide them both by 2
and repeat:

= 12 = 11

4

2

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Chapter 7: Fractions and the Big Four 113

9. Add 7⁄

10.

9 and 8⁄9.

Solve 3⁄7 + 4⁄11.

Solve It

Solve It

11. Find 5⁄

12.

6 + 7⁄10.

Add 8⁄9 + 17⁄18.

Solve It

Solve It

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114 Part II: Slicing Things Up: Fractions, Decimals, and Percents

13. Find 12⁄

14.

13 + 9⁄14.

Add 9⁄10 and 47⁄50.

Solve It

Solve It

15. Find the sum of 3⁄

16.

17 and 10⁄19.

Add 3⁄11 + 5⁄99.

Solve It

Solve It

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Chapter 7: Fractions and the Big Four 115

The Other Common Denominator:

Subtracting Fractions

As with addition, subtracting fractions that have the same denominator (also called a common denominator) is very simple: Just subtract the second numerator from the first and keep the denominator the same. In some cases, you may have to reduce the answer to lowest terms.

Subtracting fractions that have different denominators takes a bit more work. You need to increase the terms of one or both fractions so both fractions have the same denominator.

The easiest way to do this is to use cross-multiplication:

1. Cross-multiply the two fractions (as I show you in Chapter 6) and create two fractions that have a common denominator.

2. Subtract the results from Step 1.

When one denominator is a factor of the other, you can use a quick trick to find a common denominator: Increase only the terms of the fraction with the lower denominator to make both denominators the same.

Q. Find 5⁄

Q.

6 – 1⁄6.

Find 6⁄7 – 17⁄28.

A. 2⁄

A. 1

3. The denominators are both 6, so sub-

⁄4. The denominators are different, but
tract the numerators (5 and 1) to get the
because 28 is a multiple of 7, you can use
new numerator, and keep the denomina-
the quick trick described earlier. Increase
tor the same:

the terms of 6⁄7 so that its denominator is

(

28; because 28 = 7 · 4, multiply both the

− )

5

1

5 1

4

− =

=
numerator and denominator by 4:

6

6

6

6

( ⋅ )

6

6 4

24

The numerator and denominator are

=

7

(7⋅4 28

) =
both even numbers, so you can reduce
the fraction by a factor of 2:

Now both fractions have the same
denominator, so subtract the numerators

= 2
and keep the same denominator:

3

24

17

(24−17)

=

−

=

= 7

28

28

28

28

Both the numerator and denominator
are divisible by 7, so you can reduce this
fraction by a factor of 7:

= 14

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116 Part II: Slicing Things Up: Fractions, Decimals, and Percents

17. Subtract 7⁄

18.

10 – 3⁄10.

Find 4⁄5 – 1⁄3.

Solve It

Solve It

19. Solve 5⁄

20.

6 – 7⁄12.

Subtract 10⁄11 – 4⁄7.

Solve It

Solve It

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Chapter 7: Fractions and the Big Four 117

21. Solve 1⁄

22.

4 – 5⁄22.

Find 13⁄15 – 14⁄45.

Solve It

Solve It

23. Subtract 11⁄

24.

12 – 73⁄96.

What is 1⁄999 – 1⁄1,000?

Solve It

Solve It

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118 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Multiplying and Dividing Mixed Numbers

To multiply or divide two mixed numbers, convert both to improper fractions (as I show you in Chapter 6); then multiply or divide them just like any other fractions. At the end, you may have to reduce the result to lowest terms or convert the result back to a mixed number.

Q. What is 21⁄

Q.

5 · 3 1⁄4?

What is 3 1⁄2 ÷ 1 1⁄7?

A. 73⁄

A.

20. First, convert both mixed numbers

3 1⁄16. First, convert both mixed numbers
to improper fractions. Multiply the whole
to improper fractions:

number by the denominator and add the
numerator; then place your answer over

(3⋅2+1)

3 1⁄
the original denominator:

2 = =

7⁄2

2

(2⋅5+1)

( ⋅ + )

2 1⁄

1 7 1

5 = =

11⁄5

5

1 1⁄7 = =

8⁄7

7

(3⋅4+1)

Now divide the two fractions:

3 1⁄4 = =

13⁄4

4

7

8

7 7

49

÷ = ⋅ =

2

7

2 8

16

Now multiply the two fractions:

Because the answer is an improper frac-

( ⋅ )

tion, convert it to a mixed number:

11 13

11 13

143

⋅

=

=

5

4

5 ⋅ 4

20

3

16)49

Because the answer is an improper frac-
tion, change it back to a mixed number:

48

−

1

7

20)143

The final answer is 3 1⁄16.

140

−

3

The final answer is 7 3⁄20.

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Chapter 7: Fractions and the Big Four 119

25. Multiply 21⁄

26.

3 by 1 3⁄7.

Find 2 2⁄5 · 1 5⁄6.

Solve It

Solve It

27. Multiply 44⁄

28.

5 by 3 1⁄8.

Solve 4 1⁄2 ÷ 1 5⁄8.

Solve It

Solve It

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120 Part II: Slicing Things Up: Fractions, Decimals, and Percents

29. Divide 21⁄

30.

10 by 2 1⁄4.

Find 1 2⁄7 ÷ 6 3⁄10.

Solve It

Solve It

Carried Away: Adding Mixed Numbers

Adding mixed numbers isn’t really more difficult than adding fractions. Here’s how the process works:

1. Add the fractional parts, reducing the answer if necessary.

2. If the answer you found in Step 1 is an improper fraction, change it to a mixed number, write down the fractional part, and carry the whole-number part to the whole-number column.

3. Add the whole-number parts (including any number you carried over).

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Chapter 7: Fractions and the Big Four 121

Q. Add 4 1⁄

Q.

8 + 2 3⁄8.

Add 5 3⁄4 + 4 7⁄9.

A. 61⁄

A.

2. To start out, set up the problem in

10 19⁄36. To start out, set up the problem in
column form:

column form.

4 1

5 3

8

4

2 3

+

4 7

+

8

9

Add the fractions and reduce the result:

To add the fractional parts, change the

1

3

4

1

+ = =
two denominators to a common denomi-

8

8

8

2
nator using cross-multiplication. The new
numerators are 3 · 9 = 27 and 7 · 4 = 28,

Because this result is a proper fraction,
and the new denominators are 4 · 9 = 36:

you don’t have to worry about carrying.

Next, add the whole-number parts:

4 + 2 = 6

Here’s how the problem looks in column
form:

Now you can add:

4 1

8

27

28

55

+

=

36

36

36

2 3

+ 8

Because this result is an improper frac-

6 1
tion, change it to a mixed number.

2

= 119

36

Carry the 1 from this mixed number into
the whole-number column and add:

1 + 5 + 4 = 10

Here’s how the problem looks in column
form:

5 27

36

4 28

+ 36

10 19

36

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122 Part II: Slicing Things Up: Fractions, Decimals, and Percents

31. Add 31⁄

32.

5 and 4 2⁄5.

Find 7 1⁄3 + 1 1⁄6.

Solve It

Solve It

33. Add 124⁄

34.

9 and 7 8⁄9.

Find the sum of 5 2⁄3 and 9 3⁄5.

Solve It

Solve It

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Chapter 7: Fractions and the Big Four 123

35. Add 13 6⁄

36.

7 + 2 5⁄14.

Find 21 9⁄10 + 38 3⁄4.

Solve It

Solve It

Borrowing from the Whole: Subtracting

Mixed Numbers

All right, I admit it: Most students find subtracting mixed numbers about as appealing as having their braces tightened. In this section, I attempt to make this process as painless as possible.

Subtracting mixed numbers is always easier when the denominators of the fractional parts are the same. When they’re different, your first step is always to change them to fractions that have a common denominator. (In Chapter 6, I show you two ways of doing this — use whichever way works best.)

When the two mixed numbers have the same denominator, you’re ready to subtract. To start out, I show you the simplest case. Here’s how you subtract mixed numbers when the fractional part of the first number is greater than the fractional part of the second number: 1. Find the difference of the fractional parts, reducing the result if necessary.

2. Find the difference of the whole-number parts.

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124 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Subtracting mixed numbers gets a bit trickier when you need to borrow from the whole-number column into the fraction column. This is similar to borrowing in whole-number subtraction (for a refresher on borrowing, see Chapter 1).

Here’s how you subtract mixed numbers when the fractional part of the first number is less than that of the second number:

1. Borrow 1 from the whole-number column and add it to the fraction column, turning the top fraction into a mixed number.

2. Change this new mixed number into an improper fraction.

3. Use this result to subtract in the fraction column, reducing the result if necessary.

4. Perform the subtraction in the whole-number column.

Q. Subtract 84⁄

The fractional parts already have a

5 – 6 3⁄5.

common denominator, so subtract.

A. 21⁄

Notice that 1⁄6 is less than 5⁄6, so you need

5. To start out, set up the problem in
column form:

to borrow 1 from 9:

8 4

8 91 1

5

6

−

6 3

−

3 5

5

6

Now convert the mixed number 11⁄

The fractional parts already have a

6 into
an improper fraction:

common denominator, so subtract:

4

3

1

− =

8 7

5

5

5

6

Next, subtract the whole-number parts:

3 5

− 6

8 – 6 = 2

At this point, you can subtract the frac-

Here’s how the completed problem
tional parts and reduce:

looks:

7

5

2

1

− = =

6

6

6

3

8 4

5

Next, subtract the whole-number parts:

6 3

− 5

8 – 3 = 5

2 1

Here’s how the completed problem

5
looks:

Q. Subtract 91⁄6 – 35⁄6.

8 7

6

A. 51⁄3. To start out, set up the problem in

3 5

−
column form:

6

5 1

9 1

3

6

3 5

− 6

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Chapter 7: Fractions and the Big Four 125

Q. Subtract 19 4⁄

Because 32⁄88 is less than 33⁄88 , you need to

11 – 6 3⁄8.

borrow before you can subtract:

A. 12 87⁄88. To start out, set up the problem in
column form:

18191 32

88

19 4

6 33

−

11

88

6 3

− 8

Now turn the mixed number 1 32⁄88 into an
improper fraction:

The fractional parts have different de-
nominators, so change them to a common

18 120
denominator using cross-multiplication.

88

The new numerators are 4 · 8 = 32 and

6 33

−

3 · 11 = 33, and the new denominators

88
are 11 · 8 = 88:

At this point, you can subtract the frac-

4

3
tional parts and the whole-number parts:

11

8

↓

↓

18 120

88

32

33

=

=

88

88

6 33

− 88

Here’s how the problem looks now:

12 87

88

19 32

88

6 33

− 88

37. Subtract 57⁄

38.

9 – 2 4⁄9.

Find 9 1⁄8 – 7 5⁄8.

Solve It

Solve It

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126 Part II: Slicing Things Up: Fractions, Decimals, and Percents

39. Subtract 113⁄

40.

4 – 4 2⁄3.

Figure out 16 2⁄5 – 8 4⁄9.

Solve It

Solve It

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Chapter 7: Fractions and the Big Four 127

Solutions to Fractions and the Big Four

The following are the answers to the practice questions presented in this chapter.

(2⋅7)

a 2 7

14

⋅ =

.

3 9

(3⋅9 27

) =
b 3⁄8 · 6⁄11 = 9⁄44. Multiply the numerator by the numerator and the denominator by the denominator: ( ⋅ )

3

6

3 6

18

⋅

=

8 11

(8⋅11 88

) =

The numerator and denominator are both even, so both can be reduced by a factor of 2:

= 9

44
c 2⁄9 · 3⁄10 = 1⁄15. Begin by canceling out common factors. The numerator 2 and the denominator 10
are both even, so divide both by 2:

2

3

1 3

⋅

= ⋅

9 10

9 5

Next, the numerator 3 and the denominator 9 are both divisible by 3, so divide both by 3:

= 1 ⋅ 1

3 5

Now multiply straight across:

1

( ⋅1)

= =

1⁄15

3

( ⋅5)

Because you canceled out all common factors before multiplying, this answer is already reduced.

d 9⁄14 · 8⁄15 = 12⁄35. Start by canceling out common factors. The numbers 14 and 8 are both divisible by 2, and 9 and 15 are divisible by 3:

9

8

⋅

14 15

= 9 ⋅ 4

7 15

= 3 ⋅ 4

7 5

Now multiply:

(3⋅4)

= (

12

7 ⋅ 5) = 35
e 1⁄4 ÷ 6⁄7 = 7⁄24. First, change the problem to multiplication, multiplying by the reciprocal of the second fraction:

1

6

1 7

÷ = ⋅

4

7

4 6

Now complete the problem using fraction multiplication:

(1⋅7)

=

7

(4⋅6) = 24

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128 Part II: Slicing Things Up: Fractions, Decimals, and Percents
f 3⁄5 ÷ 9⁄10 = 2⁄5. Change the problem to multiplication, using the reciprocal of the second fraction: 3

9

3 10

÷

= ⋅

5

10

5 9

Complete the problem using fraction multiplication:

(3⋅10)

=

30

(5⋅9) = 45

Both the numerator and denominator are divisible by 5, so reduce the fraction by this factor:

= 6

15

They’re still both divisible by 3, so reduce the fraction by this factor:

= 25
g 8⁄9 ÷ 3⁄10 = 226⁄27. Change the problem to multiplication, using the reciprocal of the second fraction: 8

3

8 10

÷

= ⋅

9

10

9

3

Complete the problem using fraction multiplication:

(8⋅10)

=

80

(9⋅3) = 27

The numerator is greater than the denominator, so change this improper fraction to a mixed number:

= 2 26

27
h 14⁄15 ÷ 7⁄12 = 13⁄5. Change the problem to multiplication, using the reciprocal of the second fraction: 14

7

14 12

÷

=

⋅

15

12

15 7

Complete the problem using fraction multiplication:

(14⋅12)

=

168

(15⋅7) = 105

The numerator is greater than the denominator, so change this improper fraction to a mixed number:

= 1 63

105

Now the numerator and the denominator are both divisible by 3, so reduce the fractional part of this mixed number by a factor of 3:

= 1 21

35

The numerator and denominator are now both divisible by 7, so reduce the fractional part by this factor:

= 135
i 7⁄9 + 8⁄9 = 12⁄3. The denominators are the same, so add the numerators: 7

8

15

+ =

9

9

9

Both the numerator and denominator are divisible by 3, so reduce the fraction by 3:

= 53

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Chapter 7: Fractions and the Big Four 129

The result is an improper fraction, so convert it to a mixed number:

= 123
j 3⁄7 + 4⁄11 =61⁄77. The denominators are different, so change them to a common denominator by cross-multiplying. The new numerators are 3 · 11 = 33 and 4 · 7 = 28, and the new denominators are 7 · 11 = 77:

Now you can add:

33

28

61

+

=

77

77

77
k 5⁄6 + 7⁄10 = 18⁄15. The denominators are different, so change them to a common denominator by cross-multiplying. The new numerators are 5 · 10 = 50 and 7 · 6 = 42, and the new denominators are 6 · 10 = 60:

Now you can add:

50

42

92

+

=

60

60

60

Both the numerator and denominator are even, so reduce the fraction by 2:

= 46

30

They’re still both even, so reduce again by 2:

= 23

15

The result is an improper fraction, so change it to a mixed number:

= 1 8

15
l 8⁄9 + 17⁄18 = 15⁄6. The denominators are different, but 18 is a multiple of 9, so you can use the quick trick. Increase the terms of 8⁄9 so that the denominator is 18, multiplying both the numerator and denominator by 2:

( ⋅ )

8

8 2

16

=

9

(9⋅2 18

) =

Now both fractions have the same denominator, so add the numerators and keep the same denominator:

= 16 + 17 = 33

18

18

18

Both the numerator and denominator are divisible by 3, so reduce the fraction by 3:

= 11

6

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130 Part II: Slicing Things Up: Fractions, Decimals, and Percents

The result is an improper fraction, so change it to a mixed number:

= 156
m 12⁄13 + 9⁄14 = 1103⁄182. The denominators are different, so give the fractions a common denominator by cross-multiplying. The new numerators are 12 · 14 = 168 and 9 · 13 = 117, and the new denominators are 13 · 14 = 182:

Now you can add:

168

117

285

+

=

182

182

182

The result is an improper fraction, so change it to a mixed number:

= 1103

182
n 9⁄10 + 47⁄50 = 121⁄25. The denominators are different, but 50 is a multiple of 10, so you can use the quick trick. Increase the terms of 9⁄10 so that the denominator is 50, multiplying both the numerator and denominator by 5:

( )

9

9 ⋅ 5

45

=

10

(10⋅5 50

) =

Now both fractions have the same denominator, so add the numerators:

= 45 + 47 = 92

50

50

50

Both the numerator and denominator are even, so reduce the fraction by 2:

= 46

25

The result is an improper fraction, so change it to a mixed number:

= 1 21

25
o 3⁄17 + 10⁄19 = 227⁄323. The denominators are different, so change them to a common denominator by cross-multiplying. The new numerators are 3 · 19 = 57 and 10 · 17 = 170, and the new denominators are 17 · 19 = 323:

Now you can add:

57

170

227

+

=

323

323

323
p 3⁄11 + 5⁄99 = 32⁄99. The denominators are different, but 99 is a multiple of 11, so you can use the quick trick. Increase the terms of 3⁄11 so that the denominator is 99, multiplying both the numerator and denominator by 9:

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Chapter 7: Fractions and the Big Four 131

( ⋅ )

3

3 9

27

=

11

(11⋅9 99

) =

Now you can add:

27

5

32

+

=

99

99

99
q 7⁄10 – 3⁄10 = 2⁄5. The denominators are the same, so subtract the numerators and keep the same denominator:

7

3

4

−

=

10

10

10

The numerator and denominator are both even, so reduce this fraction by a factor of 2:

= 25
r 4⁄5 – 1⁄3 = 7⁄15. The denominators are different, so change them to a common denominator by cross-multiplying. The new numerators are 4 · 3 = 12 and 1 · 5 = 5, and the new denominators are 5 · 3 = 15:

Now you can subtract:

12

5

7

−

=

15

15

15
s 5⁄6 – 7⁄12 = 1⁄4. The denominators are different, but 6 is a factor of 12, so you can use the quick trick.

Increase the terms of 5⁄6 so that the denominator is 12, multiplying both the numerator and the denominator by 2:

( ⋅ )

5

5 2

10

=

6

(6⋅2 12

) =

Now the two fractions have the same denominator, so you can subtract easily: 10

7

3

−

=

12

12

12

The numerator and denominator are both divisible by 3, so reduce the fraction by a factor of 3:

= 14
t 10⁄11 – 4⁄7 = 26⁄77. The denominators are different, so change them to a common denominator by cross-multiplying. The new numerators are 10 · 7 = 70 and 4 · 11 = 44, and the new denominators are 11 · 7 = 77:

Now you can subtract:

70

44

26

−

=

77

77

77

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132 Part II: Slicing Things Up: Fractions, Decimals, and Percents
u 1⁄4 – 5⁄22 = 1⁄44. The denominators are different, so change them to a common denominator by cross-multiplying. The new numerators are 1 · 22 = 22 and 5 · 4 = 20, and the new denominators are 4 · 22 = 88:

Now you can subtract:

22

20

2

−

=

88

88

88

The numerator and denominator are both even, so reduce this fraction by a factor of 2:

= 1

44
v 13⁄15 – 14⁄45 = 5⁄9. The denominators are different, but 45 is a multiple of 15, so you can use the quick trick. Increase the terms of 13⁄15 so that the denominator is 45, multiplying the numerator and denominator by 3:

( ⋅ )

13

13 3

39

=

15

(15⋅3 45

) =

Now the two fractions have the same denominator, so you can subtract easily: 39

14

25

−

=

45

45

45

The numerator and denominator are both divisible by 5, so reduce the fraction by a factor of 5:

= 59
w 11⁄12 – 73⁄96 = 5⁄32. The denominators are different, but 96 is a multiple of 12, so you can use the quick trick. Increase the terms of 11⁄12 so that the denominator is 96 by multiplying the numerator and denominator by 8:

( ⋅ )

11

11 8

88

=

12

(12⋅8 96

) =

Now you can subtract:

88

73

15

−

=

96

96

96

Both the numerator and denominator are divisible by 3, so reduce the fraction by a factor of 3:

= 5

32
x 1⁄999 – 1⁄1,000 = 1⁄999,000. The denominators are different, so change them to a common denominator by cross-multiplying:

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Chapter 7: Fractions and the Big Four 133

Now you can subtract:

1,000

999

1

−

=

999,000

999,000

999,000
y 21⁄3 · 13⁄7 = 31⁄3. Change both mixed numbers to improper fractions: (2⋅3+1)

2 1

7

=

=

3

3

3

(1⋅7+ 3)

1 3

10

=

=

7

7

7

Set up the multiplication:

7 10

⋅

3

7

Before you multiply, you can cancel out 7s in the numerator and denominator:

= 1 ⋅ 10 = 10

3

1

3

Because the answer is an improper fraction, change it to a mixed number:

3

3)10

9

−

1

So the final answer is 31⁄3.

A 22⁄5 · 15⁄6 = 42⁄5. Change both mixed numbers to improper fractions: (2⋅5+ 2)

2 2

12

=

=

5

5

5

(1⋅6+ 5)

1 5

11

=

=

6

6

6

Set up the multiplication:

12 11

⋅

5

6

Before you multiply, you can cancel out 6s in the numerator and denominator:

= 2 ⋅ 11 = 22

5

1

5

Because the answer is an improper fraction, change it to a mixed number:

4

5)22

20

−

2

The final answer is 4 2⁄5.

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134 Part II: Slicing Things Up: Fractions, Decimals, and Percents

B 44⁄5 · 31⁄8 = 15. Change both mixed numbers to improper fractions:

(4⋅5+ 4)

4 4

24

=

=

5

5

5

(3⋅8 +1)

3 1

25

=

=

8

8

8

Set up the multiplication:

24 25

⋅

5

8

Before you multiply, cancel out both 5s and 8s in the numerator and denominator:

= 24 ⋅ 5 = 3 ⋅ 5 = 15

1

8

1 1

C 41⁄2 ÷ 15⁄8 = 210⁄13. Change both mixed numbers to improper fractions: (4⋅2+1)

4 1

9

=

=

2

2

2

(1⋅8 + 5)

1 5

13

=

=

8

8

8

Set up the division:

9

13

÷

2

8

Change the problem to multiplication using the reciprocal of the second fraction:

= 9 ⋅ 8

2 13

Cancel out a factor of 2 and multiply:

= 9 ⋅ 4 = 36

1 13

13

Because the answer is an improper fraction, change it to a mixed number:

= 210

13

D 21⁄10 ÷ 21⁄4 = 14⁄15. Change both mixed numbers to improper fractions: (2⋅10+1)

2 1

21

=

=

10

10

10

(2⋅4+1)

2 1

9

=

=

4

4

4

Set up the division:

21

9

÷

10

4

Change the problem to multiplication using the reciprocal of the second fraction:

= 21 ⋅ 4

10 9

Before you multiply, cancel 2s and 3s from the numerator and denominator:

= 21 ⋅ 2 = 7 ⋅ 2 = 14

5

9

5 3

15

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Chapter 7: Fractions and the Big Four 135

E 12⁄7 ÷ 63⁄10 = 10⁄49. Change both mixed numbers to improper fractions: (1⋅7+2)

1 2⁄7 = =

9⁄7

7

(6⋅10+3)

6 3⁄10 = =

63⁄10

10

Set up the division:

9

63

÷

7

10

Change the problem to multiplication using the reciprocal of the second fraction:

= 9 ⋅ 10

7 63

Before you multiply, cancel 9s from the numerator and denominator:

= 1 ⋅ 10 = 10

7

7

49

F 31⁄5 + 42⁄5 = 73⁄5. Set up the problem in column form:

3 1

5

4 2

+ 5

Add the fractional parts:

1

2

3

+ =

5

5

5

Because this result is a proper fraction, you don’t have to worry about carrying. Next, add the whole-number parts:

3 + 4 = 7

Here’s how the completed problem looks:

3 1

5

4 2

+ 5

7 3

5

G 71⁄3 + 11⁄6 = 81⁄2. To start out, set up the problem in column form: 7 1

3

1 1

+ 6

Next, add the fractional parts. The denominators are different, but 3 is a factor of 6, so you can use the quick trick. Increase the terms of 1⁄3 so that the denominator is 6 by multiplying the numerator and denominator by 2:

1

2

=

3

6

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136 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Now you can add and reduce the result:

2

1

3

1

+ = =

6

6

6

2

Because this result is a proper fraction, you don’t have to worry about carrying. Next, add the whole-number parts:

7 + 1 = 8

Here’s how the completed problem looks:

7 2

6

1 1

+ 6

8 1

2

H 124⁄9 + 78⁄9 = 201⁄3. Set up the problem in column form:

12 4

9

7 8

+ 9

Add the fractional parts and reduce the result:

4

8

12

4

+ =

=

9

9

9

3

Because this result is an improper fraction, convert it to a mixed number:

= 113

Carry the 1 from this mixed number into the whole-number column and add:

1 + 12 + 7 = 20

Here’s how the completed problem looks:

1

12 4

9

7 8

+ 9

20 1

3

I 52⁄3 + 93⁄5 = 154⁄15. Set up the problem in column form:

5 2

3

9 3

+ 5

Start by adding the fractional parts. Because the denominators are different, change them to a common denominator by cross-multiplying. The new numerators are 2 · 5 = 10 and 3 · 3 = 9, and the new denominators are 3 · 5 = 15:

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Chapter 7: Fractions and the Big Four 137

2

3

3

5

↓

↓

10

9

=

=

15

15

Now you can add:

10

9

19

+

=

15

15

15

Because this result is an improper fraction, convert it to a mixed number:

= 1 4

15

Carry the 1 from this mixed number into the whole-number column and add:

1 + 5 + 9 = 15

Here’s how the completed problem looks:

1

5 10

15

9 9

+ 15

15 4

15

J 136⁄7 + 25⁄14 =163⁄14. Set up the problem in column form:

13 6

7

2 5

+ 14

Begin by adding the fractional parts. Because the denominator 7 is a factor of the denominator 14, you can use the quick trick. Increase the terms of 6⁄7 so that the denominator is 14 by multiplying the numerator and denominator by 2:

6

12

=

7

14

Now you can add:

12

5

17

+

=

14

14

14

Because this result is an improper fraction, convert it to a mixed number:

= 13⁄14

Carry the 1 from this mixed number into the whole-number column and add:

1 + 13 + 2 = 16

Here’s how the completed problem looks:

1

13 12

14

2 5

+ 14

16 3

14

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138 Part II: Slicing Things Up: Fractions, Decimals, and Percents

K 219⁄10 + 383⁄4 = 6013⁄20. Set up the problem in column form: 21 9

10

38 3

+ 4

To add the fractional parts, change the denominators to a common denominator by using cross-multiplication. The new numerators are 9 · 4 = 36 and 3 · 10 = 30, and the new denominators are 10 · 4 = 40:

Now you can add:

36

30

66

+

=

40

40

40

The numerator and denominator are both even, so reduce this fraction by a factor of 2:

= 33

20

Because this result is an improper fraction, convert it to a mixed number:

= 113

20

Carry the 1 from this mixed number into the whole-number column and add:

1 + 21 + 38 = 60

Here’s how the completed problem looks:

1

21 36

40

38 30

+ 40

60 13

20

L 57⁄9 – 24⁄9 = 31⁄3. Set up the problem in column form:

5 7

9

2 4

− 9

Subtract the fractional parts and reduce:

7

4

3

1

− = =

9

9

9

3

Subtract the whole-number parts:

5 – 2 = 3

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Chapter 7: Fractions and the Big Four 139

Here’s how the completed problem looks:

5 7

9

2 4

− 9

3 1

3

M 91⁄8 – 75⁄8 = 11⁄2. Set up the problem in column form:

9 1

8

7 5

− 8

The first fraction (1⁄8) is less than the second fraction (5⁄8), so you need to borrow 1 from 9 before you can subtract:

8 91 1

8

7 5

− 8

Change the mixed number 11⁄8 to an improper fraction:

8 9

8

7 5

− 8

Now you can subtract the fractional parts and reduce:

9

5

4

1

− = =

8

8

8

2

Subtract the whole-number parts:

8 – 7 = 1

Here’s how the completed problem looks:

8 9

8

7 5

− 8

1 1

2

N 113⁄4 – 42⁄3 = 71⁄12. Set up the problem in column form:

11 3

4

4 2

− 3

The denominators are different, so get a common denominator using cross-multiplication. The new numerators are 3 · 3 = 9 and 2 · 4 = 8, and the new denominators are 4 · 3 = 12: 12_288177-ch07.qxp 7/28/08 1:07 PM Page 140

140 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Because 9⁄12 is greater than 8⁄12, you don’t need to borrow before you can subtract fractions: 11 9

12

4 8

− 12

7 1

12

O 16 2⁄5 – 84⁄9 = 7 43⁄45. To start out, set up the problem in column form: 16 2

5

8 4

− 9

The denominators are different, so find a common denominator using cross-multiplication. The new numerators are 2 · 9 = 18 and 4 · 5 = 20, and the new denominators are 5 · 9 = 45: Because 18⁄45 is less than 20⁄45, you need to borrow 1 from 16 before you can subtract fractions: 1516118

45

8 20

− 45

Change the mixed number 1 18⁄45 to an improper fraction:

15 63

45

8 20

− 45

Now you can subtract:

15 63

45

8 20

− 45

7 43

45

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Chapter 8

Getting to the Point with Decimals

In This Chapter

ᮣ Understanding how place value works with decimals

ᮣ Moving the decimal point to multiply or divide a decimal by a power of ten

ᮣ Rounding decimals to a given decimal place

ᮣ Applying the Big Four operations to decimals

ᮣ Converting between fractions and decimals

Decimals, like fractions, are a way to represent parts of the whole — that is, positive numbers less than 1. You can use a decimal to represent any fractional amount.

Decimals are commonly used for amounts of money, so you’re probably familiar with the decimal point (.), which indicates an amount smaller than one dollar.

In this chapter, I first get you up to speed on some basic facts about decimals. Then I show you how to do basic conversions between fractions and decimals. After that, you find out how to apply the Big Four operations (adding, subtracting, multiplying, and dividing) to decimals. To end the chapter, I make sure you understand how to convert any fraction to a decimal and any decimal to a fraction. This includes filling you in on the difference between a terminating decimal (a decimal with a limited number of digits) and a repeating decimal (a decimal that repeats a pattern of digits endlessly).

Getting in Place: Basic Decimal Stuff

Decimals are easier to work with than fractions because they resemble whole numbers much more than fractions do. Decimals use place value in a similar way to whole numbers. In a decimal, however, each place represents a part of the whole. Take a look at the following chart.

Thousands

Hundreds

Tens

Ones

Decimal Point

Tenths

Hundredths

Thousandths

.

Notice that the name of each decimal place, from left to right, is linked with the name of a different fraction: tenths (1⁄10), hundredths (1⁄100), thousandths (1⁄1,000), and so on.

You can use this chart to expand a decimal out as a sum. Expanding a decimal gives you a better sense of how that decimal is put together. For example, 12.011 is equal to 10 + 2 + 0⁄10 +

1⁄100 + 1⁄1,000.

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142 Part II: Slicing Things Up: Fractions, Decimals, and Percents

In a decimal, any 0 to the right of every nonzero digit is called a trailing zero. For example, in the decimal 0.070, the last zero is a trailing zero. You can safely drop this zero without changing the value of the decimal. However, the first 0 after the decimal point — which is trapped between the decimal point and a nonzero number — is a placeholding zero, which you can’t drop.

You can express any whole number as a decimal simply by attaching a decimal point and a trailing zero to the end of it. For example,

7 = 7.0

12 = 12.0

1,568 = 1,568.0

In Chapter 2, I introduce the powers of ten: 1, 10, 100, 1,000, and so forth. Moving the decimal point to the right is the same as multiplying that decimal by a power of 10. For example, ߜ Moving the decimal point one place to the right is the same as multiplying by 10.

ߜ Moving the decimal point two places to the right is the same as multiplying by 100.

ߜ Moving the decimal point three places to the right is the same as multiplying by 1,000.

Similarly, moving the decimal point to the left is the same as dividing that decimal by a power of 10. For example,

ߜ Moving the decimal point one place to the left is the same as dividing by 10.

ߜ Moving the decimal point two places to the left is the same as dividing by 100.

ߜ Moving the decimal point three places to the left is the same as dividing by 1,000.

To multiply a decimal by any power of 10, count the number of zeros and move the decimal point that many places to the right. To divide a decimal by any power of 10, count the number of zeros and move the decimal point that many places to the left.

Rounding decimals is similar to rounding whole numbers (if you need a refresher, see Chapter 1). Generally speaking, to round a number to a given decimal place, focus on that decimal place and the place to its immediate right; then round as you would with whole numbers:

ߜ Rounding down: If the digit on the right is 0, 1, 2, 3, or 4, just drop this digit and every digit to its right.

ߜ Rounding up: If the digit on the right is 5, 6, 7, 8, or 9, add 1 to the digit in the first decimal place and then drop every digit to its right.

When rounding, people often refer to the first three decimal places in two different ways —
by the number of the decimal place and by the the name:

ߜ Rounding to one decimal place is the same as rounding to the nearest tenth.

ߜ Rounding to two decimal places is the same as rounding to the nearest hundredth.

ߜ Rounding to three decimal places is the same as rounding to the nearest thousandth.

When rounding to four or more decimal places, the names get longer, so they’re usually not used.

13_288177-ch08.qxp 7/28/08 1:04 PM Page 143

Chapter 8: Getting to the Point with Decimals 143

Q. Expand the decimal 7,358.293.

Q. Multiply 3.458 · 100.

A. 7,358293

.

= 7,000 + 300 + 50 + 8 +

A. 345.8. The number 100 has two zeros, so

2

9

3

+

+
to multiply by 100, move the decimal

10

100

1,000
point two places to the right.

Q. Simplify the decimal 0400.0600 by remov-
ing all leading and trailing zeros, without

Q. Divide 29.81 ÷ 10,000.

removing placeholding zeros.

A.

A. 0.002981. The number 10,000 has four

400.06. The first 0 is a leading zero
zeros, so to divide by 10,000, move the
because it appears to the left of all
decimal point four places to the left.

nonzero digits. The last two 0s are trail-
ing zeros because they appear to the
right of all nonzero digits. The remaining
three zeros are placeholding zeros.

1. Expand the following decimals:

2. Simplify each of the following decimals by
remove all leading and trailing zeros when-
a. 2.7
ever possible, without removing placehold-
b. 31.4
ing zeros:

c. 86.52
a. 5.80
d. 103.759
b. 7.030
e. 1,040.0005
c. 90.0400
f. 16,821.1384
d. 9,000.005

Solve It
e. 0108.0060
f. 00100.0102000

Solve It

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144 Part II: Slicing Things Up: Fractions, Decimals, and Percents

3. Do the following decimal multiplication

4. Round each of the following decimals to
problems by moving the decimal point the
the number of places indicated:

correct number of places:

a. Round 4.777 to one decimal place.

a. 7.32 · 10
b. Round 52.305 to the nearest tenth.

b. 9.04 · 100
c. Round 191.2839 to two decimal places.

c. 51.6 · 100,000
d. Round 99.995 to the nearest hundredth.

d. 183 ÷ 100
e. Round 0.00791 to three decimal places.

e. 2.786 ÷ 1,000
f. Round 909.9996 to the nearest
f. 943.812 ÷ 1,000,000
thousandth.

Solve It

Solve It

Knowing Simple Decimal-Fraction Conversions

Some conversions between decimals and fractions are easy to do. The conversions in Table 8-1
are all so common that they’re worth memorizing. You can also use these to convert some decimals greater than one to mixed numbers, and vice versa.

Table 8-1

Equivalent Decimals and Fractions

Tenths

Eighths

Fifths

Quarters

Half

0.1 = 1⁄10

0.125 = 1⁄8

0.2 = 1⁄5

0.25 = 1⁄4

0.3 = 3⁄10

0.375 = 3⁄8

0.4 = 2⁄5

0.5 = 1⁄2

0.625 = 5⁄8

0.6 = 3⁄5

0.7 = 7⁄10

0.75 = 3⁄4

0.875 = 7⁄8

0.8 = 4⁄5

0.9 = 9⁄10

13_288177-ch08.qxp 7/28/08 1:04 PM Page 145

Chapter 8: Getting to the Point with Decimals 145

Q. Convert 13.7 to a mixed number.

Q. Change 94⁄5 to a decimal.

A. 137⁄

A.

10. The whole-number part of the deci-

9.8. The whole-number part of the mixed
mal (13) becomes the whole-number part
number (9) becomes the whole-number
of the mixed number. Use the conversion
part of the decimal. Use the conversion
chart to change the rest of the decimal
chart to change the fractional part of the

(0.7) to a fraction.

mixed number (4/5) to a decimal.

5. Convert the following decimals into

6. Change these fractions to decimals:

fractions:

a. 9⁄10
a. 0.7
b. 2⁄5
b. 0.4
c. 3⁄4
c. 0.25
d. 3⁄8
d. 0.125
e. 7⁄8
e. 0.1
f. 1⁄2
f. 0.75

Solve It

Solve It

7. Change these decimals to mixed numbers:

8. Change these mixed numbers to decimals:

a. 1.6
a. 1 1⁄5
b. 3.3
b. 2 1⁄10
c. 14.5
c. 3 1⁄2
d. 20.75
d. 5 1⁄4
e. 100.625
e. 7 1⁄8
f. 375.375
f. 12 5⁄8

Solve It

Solve It

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146 Part II: Slicing Things Up: Fractions, Decimals, and Percents

A New Lineup: Adding and

Subtracting Decimals

You shouldn’t lose much sleep at night worrying about adding and subtracting decimals, because it’s nearly as easy as adding and subtracting whole numbers. Simply line up the decimal points and then add or subtract just as you would with whole numbers. The decimal point drops straight down in your answer.

To avoid mistakes (and to make your teacher happy), make sure your columns are neat. If you find it helpful, fill out the columns on the right with trailing zeros so that all numbers have the same number of decimal places. You may need to add these trailing zeros if you’re subtracting a decimal from a number that has fewer decimal places.

Q. Add the following decimals: 321.81 +

Q. Subtract the following decimals:

24.5 + 0.006 = ?

978.245 – 29.03 = ?

A. 346.316. Place the decimal numbers in a

A. 949.215. Place the decimals one on top
column (as you would for column addi-
of the other with the decimal points lined
tion) with the decimal points lined up:

up, dropping the decimal point straight
down in the answer:

321 810

.

24 500

.

978 245

.

−

0

+ 006

.

29 030

.

.

.

Notice that the decimal point in the

Now subtract as you would when sub-
answer lines up with the others. As you
tracting whole numbers, borrowing when
can see, I’ve also filled out the columns
necessary (see Chapter 2 for more on
with trailing zeros. This is optional, but
borrowing in subtraction):

do it if it helps you to see how the

6 1
columns line up.

9 7 8 245

.

29

− 030

.

Now add as you would when adding
whole numbers, carrying when neces-

949 215

.

sary (see Chapter 2 for more on carrying
in addition):

1

321 810

.

24 500

.

0

+ 006

.

346 316

.

13_288177-ch08.qxp 7/28/08 1:04 PM Page 147

Chapter 8: Getting to the Point with Decimals 147

9. Add these decimals: 17.4 + 2.18 = ?

10. Solve the following decimal addition:

0.0098 + 10.101 + 0.07 + 33 =

Solve It

Solve It

11. Add the following decimals: 1,000.001 +

12. Subtract these decimals: 0.748 – 0.23 = ?

75 + 0.03 + 800.2 = ?

Solve It

Solve It

13. Solve the following: 674.9 – 5.0001.

14. Find the solution to this decimal subtrac-
tion problem: 100.009 – 0.68 = ?

Solve It

Solve It

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148 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Counting Decimal Places: Multiplying Decimals

To multiply two decimals, don’t worry about lining up the decimal points. In fact, to start out, ignore the decimal points. Here’s how the multiplication works:

1. Perform the multiplication just as you would for whole numbers.

2. When you’re done, count the number of digits to the right of the decimal point in
each factor and add the results.

3. Place the decimal point in your answer so that your answer has the same number of digits after the decimal point.

Even if the last digit in the answer is 0, you still need to count this as a digit when placing the decimal point in a multiplication problem. After the decimal point is in place, however, you can drop trailing zeros.

Q. Multiply the following decimals:

At this point, you’re ready to find out

74.2 · 0.35 = ?

where the decimal point goes in the
answer. Count the number of decimal

A. 25.97. Ignoring the decimal points, per-
places in the two factors (74.2 and 0.35),
form the multiplication just as you would
add these two numbers together (1 + 2 =
for whole numbers:

3), and place the decimal point in the
answer so that it has three digits after

74 2

.

the decimal point:

0

× 3

. 5

74 2

.

← 1 digit after deccimal

3710

0

× 3

. 5 ← 2 digits after decimal

22260

+

3710

25970

22260

+

25 970

.

← 1+ 2 = 3 digits after decimal

15. Multiply these decimals: 0.635 · 0.42 = ?

16. Perform the following decimal multiplica-
tion: .675 · 34.8 = ?

Solve It

Solve It

13_288177-ch08.qxp 7/28/08 1:04 PM Page 149

Chapter 8: Getting to the Point with Decimals 149

17. Solve the following multiplication problem:

18. Find the solution to this decimal multiplica-

943 · 0.0012 = ?

tion: 1.006 · 0.0807 = ?

Solve It

Solve It

Decimal Points on the Move: Dividing Decimals

Dividing decimals is similar to dividing whole numbers, except you have to handle the decimal point before you start dividing. Here’s how to divide decimals step by step: 1. Move the decimal point in the divisor and dividend.

Turn the divisor (the number you’re dividing by) into a whole number by moving the decimal point all the way to the right. At the same time, move the decimal point in the dividend (the number you’re dividing) the same number of places to the right.

2. Place a decimal point in the quotient (the answer) directly above where the decimal point now appears in the dividend.

3. Divide as usual, being careful to line up the quotient properly so that the decimal point falls into place.

Line up each digit in the quotient just over the last digit in the dividend used in that cycle. Flip to Chapter 1 if you need a refresher on long division.

As with whole-number division, sometimes decimal division doesn’t work out evenly at the end. With decimals, however, you never write a remainder. Instead, attach enough trailing zeros to round the quotient to a certain number of decimal places. The digit to the right of the digit you’re rounding to tells you whether to round up or down, so you always have to figure out the division to one extra place (see “Getting in Place: Basic Decimal Stuff” earlier in this chapter for more on how to round decimals). See the following chart:

To Round a Decimal to

Fill Out the Dividend with Trailing Zeros to

A whole number

One decimal place

One decimal place

Two decimal places

Two decimal places

Three decimal places

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150 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Q. Divide the following: 9.152 ÷ 0.8 = ?

Q. Divide the following: 21.9 ÷ 0.015 = ?

A. 11.44. To start out, write the problem as

A. 1,460. Set up the problem as usual:

usual:

0 015

.

)21 900

.

0 8

. )9 1

. 52

Notice that I attach two trailing zeros to

Turn 0.8 into the whole number 8 by
the dividend. I do this because I need to
moving the decimal point one place to the
move the decimal points in each number
right. At the same time, move the decimal
three places to the right. Again, place the
point in 9.1526 one place to the right.

decimal point in the quotient directly

Put your decimal point in the quotient
above where it now appears in the divi-
directly above where it falls in 91.25:

dend, 21900:

.

.

8 ).91 5

. 2

15 ).21900.

Now you’re ready to divide. Just be care-

Now you’re ready to divide. Line up the
ful to line up the quotient properly so
quotient carefully so the decimal point
that the decimal point falls into place.

falls into place:

11 44

.

1460.

8 ).91 5

. 2

8 ).21900.

8

−

−

11

15

8

−

69

−

35

60

32

−

90

−

32

90

32

−

0

0

Even though the division comes out even
after you write the digit 6 in the quotient,
you still need to add a placeholding zero
so that the decimal point appears in the
correct place.

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Chapter 8: Getting to the Point with Decimals 151

19. Divide these two decimals: 9.345 ÷ 0.05 = ?

20. Solve the following divsion: 3.15 ÷ .021 = ?

Solve It

Solve It

21. Perform the following decimal division,

22. Find the solution, rounding to the nearest
rounding to one decimal place: 6.7 ÷ 10.1.

hundredth: 9.13 ÷ 4.25.

Solve It

Solve It

Changing Decimals to Fractions

Some conversions from very common decimals to fractions are easy (see “Knowing Simple Decimal-Fraction Conversions” earlier in this chapter). In other cases, you have to do a bit more work. Here’s how to change any decimal into a fraction:

1. Create a “fraction” with the decimal in the numerator and 1.0 in the denominator.

This isn’t really a fraction, because a fraction always has whole numbers in both the numerator and denominator, but you turn it into a fraction in Step 2.

When converting a decimal that’s greater than 1 to a fraction, separate out the whole-number part of the decimal before you begin; work only with the decimal part. The resulting fraction is a mixed number.

2. Move the decimal point in the numerator enough places to the right to turn the numerator into a whole number and move the decimal point in the denominator the same number of places.

3. Drop the decimal points and any trailing zeros.

4. Reduce the fraction to lowest terms if necessary.

See Chapter 6 for info on reducing fractions.

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152 Part II: Slicing Things Up: Fractions, Decimals, and Percents

A quick way to make a fraction out of a decimal is to use the name of the smallest decimal place in that decimal. For example,

ߜ In the decimal 0.3, the smallest decimal place is the tenths place, so the equivalent fraction is 3⁄10.

ߜ In the decimal 0.29, the smallest decimal place is the hundredths place, so the equivalent fraction is 29⁄100.

ߜ In the decimal 0.817, the smallest decimal place is the hundredths place, so the equivalent fraction is 817⁄1,000.

Q. Change the decimal 0.83 to a fraction.

Q. Change the decimal 0.0205 to a fraction.

A. 83⁄

A. 41

100. Create a “fraction” with 0.83 in the

⁄2,000. Create a “fraction” with 0.0205 in the
numerator and 1.0 in the denominator:

numerator and 1.0 in the denominator:

0 8

. 3

0 0205

.

1 0

.

1 0

.

Move the decimal point in 0.83 two places

Move the decimal point in the 0.0205 four
to the right to turn it into a whole number;
places to the right to turn the numerator
and move the decimal point in the denomi-
into a whole number; and move the deci-
nator the same number of places. I do this
mal point in the denominator the same
one decimal place at a time:

number of places:

0 8

. 3

8 3

.

83 0

.

=

=

0 0205

.

1 0

.

10 0

.

100 0

.

1 0

.

At this point, you can drop the decimal

0 205

.

=
points and trailing zeros in both the

10 0

.

numerator and denominator.

02 05

.

= 100 0.

020 5

.

= 1,000 0.

= 0205 0

.

10,000 0

.

Drop the decimal points, plus any lead-
ing or trailing zeros in both the numera-
tor and denominator.

= 205

10,000

Both the numerator and denominator are
divisible by 5, so reduce this fraction:

= 41

2,000

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Chapter 8: Getting to the Point with Decimals 153

23. Change the decimal 0.27 to a fraction.

24. Convert the decimal 0.0315 to a fraction.

Solve It

Solve It

25. Write 45.12 as a mixed number.

26. Change 100.001 to a mixed number.

Solve It

Solve It

Changing Fractions to Decimals

To change any fraction to a decimal, just divide the numerator by the denominator.

Often, you need to find the exact decimal value of a fraction. You can represent every fraction exactly as either a terminating decimal or a repeating decimal:

ߜ Terminating decimal: A terminating decimal is simply a decimal that has a finite (limited) number of digits. For example, the decimal 0.125 is a terminating decimal with 3
digits. Similarly, the decimal 0.9837596944883383 is a terminating decimal with 16
digits.

ߜ Repeating decimal: A repeating decimal is a decimal that repeats the same digits forever. For example, the decimal 0 7

. is a repeating decimal. The bar over the 7 means
that the number 7 is repeated forever: 0.777777777 . . . . Similarly, the
decimal 0 345

.

91 is also a repeating decimal. The bar over the 91 means that these two
numbers are repeated forever: 0.3459191919191919 . . . .

Whenever the answer to a division problem is a repeating decimal, you’ll notice a pattern developing as you divide: When you subtract, you find the same numbers showing up over and over again. When this happens, check the quotient to see whether you can spot the repeating pattern and place a bar over these numbers.

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154 Part II: Slicing Things Up: Fractions, Decimals, and Percents

When you’re asked to find the exact decimal value of a fraction, feel free to attach trailing zeros to the dividend (the number you’re dividing) as you go along. Keep dividing until the division either works out evenly (so the quotient is a terminating decimal) or a repeating pattern develops (so it’s a repeating decimal).

Q. Convert the fraction 9⁄

Q.

16 to an exact deci-

What is the exact decimal value of the
mal value.

fraction 5⁄6?

A. 0.5625. Divide 9 ÷ 16:

A. 0.83. Divide 5 ÷ 6:

16)9 000

.

Because 6 is too big to go into 5, I
attached a decimal point and some trail-

Because 16 is too big to go into 9,
ing zeros to the 5. Now divide:

I attached a decimal point and some
trailing zeros to the 9. Now you can
divide as I show you earlier in this
chapter:

0 5625

.

16)9 0000

.

80

−100

96

−40

32

−

As you can see, a pattern has developed.

80

80

−

No matter how many trailing zeros you

0
attach, the quotient will never come out
evenly. Instead, the quotient is the repeat-
ing decimal .083 . The bar over the 3 indi-
cates that the number 3 repeats forever:

0.83333333 . . . .

27. Change 13⁄

28.

16 to an exact decimal value.

Express 7⁄9 exactly as a decimal.

Solve It

Solve It

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Chapter 8: Getting to the Point with Decimals 155

Solutions to Getting to the Point
with Decimals

The following are the answers to the practice questions presented in this chapter.

a Expand the following decimals:

a. 2.7 = 2 + 7

10
b. 31.4 = 30 + 1 + 4

10
c. 86.52 = 80 + 6 + 5 + 2

10

100
d. 103.759 = 100 + 3 + 7 + 5 +

9

10

100

1,000
e. 1,040.0005 = 1,000 + 40 +

5

10,000
f. 16,821.1384 = 10,000 + 6,000 + 800 + 20 + 1 + 1 + 3 +

8

+

4

10

100

1,000

10,000
b Simplify the decimals without removing placeholding zeros:

a. 5.80 = 5.8
b. 7.030 = 7.03
c. 90.0400 = 90.04
d. 9,000.005 = 9,000.005
e. 0108.0060 = 108.006
f. 00100.0102000 = 100.0102
c Perform decimal multiplication:

a. 7.32 · 10 = 73.2
b. 9.04 · 100 = 904
c. 51.6 · 100,000 = 5,160,000
d. 183 ÷ 100 = 1.83
e. 2.786 ÷ 1,000 = 0.002786
f. 943.812 ÷ 1,000,000 = 0.000943812
d Round each decimal to the number of places indicated:

a. One decimal place: 4.777 → 4.8
b. Nearest tenth: 52.305 → 52.3
c. Two decimal places: 191.2839 → 191.28
d. Nearest hundredth: 99.995 → 100.00
e. Three decimal places: 0.00791 → 0.008
f. Nearest thousandth: 909.9996 → 910.000

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156 Part II: Slicing Things Up: Fractions, Decimals, and Percents
e Convert the decimals to fractions:

a. 0.7= 7⁄10
b. 0.4 = 2⁄5
c. 0.25 = 1⁄4
d. 0.125 = 1⁄8
e. 0.1 = 1⁄10
f. 0.75 = 3⁄4
f Change the fractions to decimals:

a. 9⁄10 = 0.9
b. 2⁄5 = 0.4
c. 3⁄4 = 0.75
d. 3⁄8 = 0.375
e. 7⁄8 = 0.875
f. 1⁄2 = 0.5
g Change the decimals to mixed numbers:

a. 1.6 = 1 3⁄5
b. 3.3 = 3 3⁄10
c. 14.5 = 14 1⁄2
d. 20.75 = 20 3⁄4
e. 100.625 = 100 5⁄8
f. 375.375 = 375 3⁄8
h Change the mixed numbers to decimals:

a. 1 1⁄5 = 1.2
b. 2 1⁄10 = 2.1
c. 3 1⁄2 = 3.5
d. 5 1⁄4 = 5.25
e. 7 1⁄8 = 7.125
f. 12 5⁄8 = 12.675
i 17.4 + 2.18 = 19.58. Place the numbers in a column as you would for addition with whole numbers, but with the decimal points lined up. I’ve filled out the columns with trailing zeros to help show how the columns line up:

17 40

.

2

+ 1

. 8

19 58

.

Notice that the decimal point in the answer lines up with the others.

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Chapter 8: Getting to the Point with Decimals 157
j 0.0098 + 10.101 + 0.07 + 33 = 43.1808. Line up the decimal points and do column addition: 1

.0098

.

10 1010

.0700

+

.

33 0000

.

43 1

1808
k 1,000.001 + 75 + 0.03 + 800.2 = 1,875.231. Place the decimal numbers in a column, lining up the decimal points:

1000 001

.

75 000

.

0 030

.

800

+

200

.

1875 231

.

l 0.748 – 0.23 = 0.518. Place the first number on top of the second number, with the decimal points lined up. I’ve also added a trailing 0 to the second number to fill out the right-hand column and emphasize how the columns line up:

0 748

.

0

− 230

.

0 518

.

Notice that the decimal point in the answer lines up with the others.

m 674.9 – 5.0001 = 669.8999. Place the first number on top of the second number, with the decimal points lined up. I’ve filled out the right-hand column with trailing zeros so I can complete the math: n 100.009 – 0.68 = 99.329. Place the first number on top of the second number, with the decimal points lined up:

o 0.635 · 0.42 = 0.2667. Place the first number on top of the second number, ignoring the decimal points. Complete the multiplication as you would for whole numbers:

0 635

.

← 3 digits after decimal

0

× 4

. 2 ← 2 digits after decimal

1270

25400

+

0 26670

.

← 3 + 2 = 5 diggits after decimal

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158 Part II: Slicing Things Up: Fractions, Decimals, and Percents

At this point, you’re ready to find out where the decimal point goes in the answer. Count the number of decimal places in the two factors, add these two numbers together (3 + 2 = 5), and place the decimal point in the answer so that it has five digits after the decimal point. After you place the decimal point (but not before!), you can drop the trailing zero.

p 0.675 · 34.8 = 23.49. Ignore the decimal points and simply place the first number on top of the second. Complete the multiplication as you would for whole numbers:

0 675

.

← 3 digits after decimal

34

× 8

.

← 1 digit after decimal

5400

27000

202500

+

23 4900

.

← 3 +1 = 4 digits after decimal

Count the number of decimal places in the two factors, add these two numbers together (3 + 1 = 4), and place the decimal point in the answer so that it has four digits after the decimal point. Last, you can drop the trailing zeros.

q 943 · 0.0012 = 1.1316. Complete the multiplication as you would for whole numbers: 943 ← 0 digits after de
ecimal

0

× 0012

.

← 4 digits after decimal

1886

9430

+

1 1316

.

← 0 + 4 = 4 digitts after decimal

Zero digits come after the decimal point in the first factor, and you have four after-decimal digits in the second factor, for a total of 4 (0 + 4 = 4); place the decimal point in the answer so that it has four digits after the decimal point.

r 1.006 · 0.0807 = 0.0811842. Complete the multiplication as you would for whole numbers: 1 006

.

← 3 digits aafter decimal

0

× 0807

.

← 4 digits after decimal

7042

804800

+

0 0811842

.

← 3 + 4 = 7 digits after decimal

You have a total of seven digits after the decimal points in the two factors — three in the first factor and four in the second (3 + 4 = 7) — so place the decimal point in the answer so that it has seven digits after the decimal point. Notice that I need to create an extra decimal place in this case by attaching an additional nontrailing 0.

s 9.345 ÷ 0.05 = 186.9. To start out, write the problem as usual:

0 05

.

)9 345

.

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Chapter 8: Getting to the Point with Decimals 159

Turn the divisor (0.05) into a whole number by moving the decimal point two places to the right. At the same time, move the decimal point in the dividend (9.345) two places to the right.

Place the decimal point in the quotient directly above where it now appears in the dividend:

.

5 ).934 5

.

Now you’re ready to divide. Be careful to line up the quotient properly so that the decimal point falls into place.

186 9

.

5 ).934 5

.

5

−43

40

− 34

30

− 45

45

− 0
t 3.15 ÷ 0.021 = 150. Write the problem as usual:

0.

)

021 3 15

.

You need to move the decimal point in the divisor (0.021) three places to the right, so attach an additional trailing zero to the dividend (3.15) to extend it to three decimal places: 0.

)

021 3 150

.

Now you can move both decimal points three places to the right. Place the decimal point in the quotient above the decimal point in the dividend:

.

21 ).3150.

Divide, being careful to line up the quotient properly:

) 150.

21 3150.

21

−105

105

− 0

Remember to insert a placeholding zero in the quotient so that the decimal point ends up in the correct place.

u 6.7 ÷ 10.1 = 0.7. To start out, write the problem as usual:

10. )

1 6 7

.

Turn the divisor (10.1) into a whole number by moving the decimal point one place to the right.

At the same time, move the decimal point in the dividend (6.7) one place to the right:

.

101 ).67.

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160 Part II: Slicing Things Up: Fractions, Decimals, and Percents

The problem asks you to round the quotient to one decimal place, so fill out the dividend with trailing zeros to two decimal places:

.

101 ).67 00

.

Now you’re ready to divide:

0 6

. 6

101 ).67 00

.

606

− 640

606

− 34

Round the quotient to one decimal place:

0.66 → 0.7
v 9.13 ÷ 4.25 = 2.15. First, write the problem as usual:

4 2

. 5)9 1

. 3

Turn the divisor (4.25) into a whole number by moving the decimal point two places to the right.

At the same time, move the decimal point in the dividend (9.13) two places to the right:

.

425)913.

The problem asks you to round the quotient to the nearest hundredth, so fill out the dividend with trailing zeros to three decimal places:

.

425 ).913 000

.

Now divide, carefully lining up the quotient:

2 148

.

425 ).913 000

.

850

− 630

425

−2050

1700

− 3500

34

− 000

100

Round the quotient to the nearest hundredth:

2.148 → 2.15
w 0.27 = 27⁄100. Create a “fraction” with 0.27 in the numerator and 1.0 in the denominator. Then move the decimal points to the right until both the numerator and denominator are whole numbers:

0 2

. 7

2 7

.

27 0

.

=

=

1 0

.

10 0

.

100 0

.

At this point, you can drop the decimal points and trailing zeros.

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Chapter 8: Getting to the Point with Decimals 161
x 0.0315 = 63⁄2,000. Create a “fraction” with 0.0315 in the numerator and 1.0 in the denominator. Then move the decimal points in both the numerator and denominator to the right one place at a time. Continue until both the numerator and denominator are whole numbers:

0.0315⁄1.0 = 0.315⁄10.0 = 3.15⁄100.0 = 31.5⁄1,000.0 = 315.0⁄10,000.0

Drop the decimal points and trailing zeros. The numerator and denominator are both divisible by 5, so reduce the fraction:

315⁄10,000 = 63⁄2,000
y 45.12 = 453⁄25. Before you begin, separate out the whole number portion of the decimal (45).

Create a “fraction” with 0.12 in the numerator and 1.0 in the denominator. Move the decimal points in both the numerator and denominator to the right until both are whole numbers: 0.12⁄1.0 = 1.2⁄10.0 = 12.0⁄100.0

Drop the decimal points and trailing zeros. As long as the numerator and denominator are both divisible by 2 (that is, even numbers), you can reduce this fraction:

12⁄100 = 6⁄50 = 3⁄25

To finish up, reattach the whole number portion that you separated at the beginning.

A 100.001 = 1001⁄1,000. Separate out the whole number portion of the decimal (100) and create a

“fraction” with 0.001 in the numerator and 1.0 in the denominator. Move the decimal points in both the numerator and denominator to the right one place at a time until both are whole numbers:

0.001⁄1.0 = 0.01⁄10.0 = 0.1⁄100.0 = 1.0⁄1,000.0

Drop the decimal points and trailing zeros and reattach the whole-number portion of the number you started with:

100 1⁄1,000

B 13⁄16 = 0.8125. Divide 13 ÷ 16, attaching plenty of trailing zeros to the 13: 0 8125

.

16)13 00000

.

128

− 20

16

− 40

32

− 80

80

− 0

This division eventually ends, so the quotient is a terminating decimal.

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162 Part II: Slicing Things Up: Fractions, Decimals, and Percents

C 7⁄9 = 0 7. . Divide 7 ÷ 9, attaching plenty of trailing zeros to the 7: A pattern has developed in the subtraction: 70 – 63 = 7, so when you bring down the next 0, you’ll get 70 again. Therefore, the quotient is a repeating decimal.

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Chapter 9

Playing the Percentages

In This Chapter

ᮣ Converting between percents and decimals

ᮣ Switching between percents and fractions

ᮣ Solving all three types of percent problems

Like fractions and decimals (which I discuss in Chapters 6, 7, and 8), percents are a way of describing parts of the whole. The word percent literally means for 100, but in practice, it means out of 100. So when I say that 50 perecent of my shirts are blue, I mean that 50
out of 100 shirts — that is, half of them — are blue. Of course, you don’t really have to own as many shirts as I claim to in order to make this statement. If you own 8 shirts and 50 percent of them are blue, then you own 4 blue shirts.

In this chapter, I show you how to convert percents to and from decimals and fractions. In the last section, I show you how to use the Percent Circle, a nifty tool for solving the three main types of percent problems. Become familiar with percents, and you’ll be able to figure out discounts, sales tax, tips for servers, and my favorite: interest on money in the bank.

Converting Percents to Decimals

Percents and decimals are very similar forms, so everything you know about decimals (Chapter 8) carries over when you’re working with percents. All you need to do is convert your percent to a decimal, and you’re good to go.

To change a whole-number percent to a decimal, simply replace the percent sign with a decimal point and then move this decimal point two places to the left; after this, you can drop any trailing zeros. Here are a few common conversions between percents and decimals: 100% = 1

75% = 0.75

50% = 0.5

25% = 0.25

20% = 0.2

10% = 0.1

Sometimes a percent already has a decimal point. In this case, just drop the percent sign and move the decimal point two places to the left. For instance, 12.5% = 0.125.

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164 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Q. Change 80% to a decimal.

Q. Change 37.5% to a decimal.

A. 0.8. Replace the percent sign with a deci-

A. 0.375. Drop the percent sign and move
mal point — changing 80% to 80 — and
the decimal point two places to the left:

then move the decimal point two places

37.5% = 0.375
to the left:

80% = 0.80

At the end, you can drop the trailing zero
to get 0.8.

1. Change 90% to a decimal.

2. A common interest rate on an investment
such as a bond is 4%. Convert 4% to a

Solve It
decimal.

Solve It

3. Find the decimal equivalent of 99.44%.

4. What is 243.1% expressed as a decimal?

Solve It

Solve It

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Chapter 9: Playing the Percentages 165

Changing Decimals to Percents

Calculating with percents is easiest when you convert to decimals first. When you’re done calculating, however, you often need to change your answer from a decimal back to a percent. This is especially true when you’re working with interest rates, taxes, or the likelihood of a big snowfall the night before a big test. All of these numbers are most commonly expressed as percents.

To change a decimal to a percent, move the decimal point two places to the right and attach a percent sign. If the result is a whole number, you can drop the decimal point.

Q. Change 0.6 to a percent.

A. 60%. Move the decimal point two places
to the right and attach a percent sign:

0.6 = 60%

5. Convert 0.57 to a percent.

6. What is 0.3 expressed as a percent?

Solve It

Solve It

7. Change 0.015 to a percent.

8. Express 2.222 as a percent.

Solve It

Solve It

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166 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Switching from Percents to Fractions

Some percents are easy to convert to fractions. Here are a few quick conversions that are worth knowing:

1% = 1⁄100

5% = 1⁄20

10% = 1⁄10

20% = 1⁄5

25% = 1⁄4

50% = 1⁄2

75% = 3⁄4

100% = 1

Beyond these simple conversions, changing a percent to a fraction isn’t a skill you’re likely to use much outside of a math class. Decimals are much easier to work with.

However, teachers often test you on this skill to make sure you understand the ins and outs of percents, so here’s the scoop on converting percents to fractions: To change a percent to a fraction, use the percent without the percent sign as the numerator (top number) of the fraction and use 100 as the denominator (bottom number). When necessary, reduce this fraction to lowest terms or change it to a mixed number. (For a refresher on reducing fractions, see Chapter 6.)

Q. Change 35% to a fraction.

You can reduce this fraction because the
numerator and denominator are both

A. 7⁄
divisible by 5:

20. Place 35 in the numerator and 100 in
the denominator:

= 7

20

35% = 35

100

9. Change 19% to a fraction.

10. A common interest rate on credit cards and
other types of loans is 8%. What is 8%

Solve It
expressed as a fraction?

Solve It

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Chapter 9: Playing the Percentages 167

11. Switch 123% to a fraction.

12. Convert 375% to a fraction.

Solve It

Solve It

Converting Fractions to Percents

Knowing how to make a few simple conversions from fractions to percents is a useful real-world skill. Here are some of the most common conversions:

1⁄100 = 1%

1⁄20 = 5%

1⁄10 = 10%

1⁄5 = 20%

1⁄4 = 25%

1⁄2 = 50%

3⁄4 = 75%

1 = 100%

Beyond these, you’re not all that likely to need to convert a fraction to a percent outside of a math class. But then, passing your math class is important, so in this section I show you how to make this type of conversion.

Converting a fraction to a percent is a two-step process:

1. Convert the fraction to a decimal, as I show you in Chapter 8.

In some problems, the result of this step may be a repeating decimal. This is fine —
in this case, the percent will also contain a repeating decimal.

2. Convert this decimal to a percent.

Move the decimal point two places to the right and add a percent sign.

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168 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Q. Change the fraction 1⁄9 to a percent.

The result is the repeating decimal 0 1

. .

Now change this repeating decimal to a

A. 111.% . First, change 1⁄9 to a decimal:

percent:

0 111

.

0 1

. = 11 1

. %

9) 1 000

.

9

−

10

9

−

10

9

−

1

13. Express 2⁄

14.

5 as a percent.

Change 3⁄20 to a percent.

Solve It

Solve It

15. Convert 7⁄

16.

8 to a percent.

Change 2⁄11 to a percent.

Solve It

Solve It

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Chapter 9: Playing the Percentages 169

Mixing Things Up with the Percent Circle

In this section, I show you how to recognize the three main types of percent problems. Then I give you a tool for solving them all: the Percent Circle.

Percent problems give you two pieces of information and ask you to find the third piece.

Here are the three pieces — and the kinds of questions that ask for each piece: ߜ The percent: The problem may give you the starting and ending numbers and ask you to find the percent. Here are some ways this problem can be asked:

?% of 4 is 1

What percent of 4 is 1?

1 is what percent of 4?

The answer is 25%, because 25% · 4 = 1.

ߜ The starting number: The problem may give you the percent and the ending number and ask you to find the starting number:

10% of ? is 40

10% of what number is 40?

40 is 10% of what number?

This time, the answer is 400, because 10% · 400 = 40.

ߜ The ending number: The most common type of percent problem gives you a percent-age and a starting number and asks you to figure out the ending number:

50% of 6 is ?

50% of 6 equals what number?

Can you find 50% of 6?

No matter how I phrase it, notice that the problem always includes 50% of 6. The answer is 3, because 50% · 6 = 3.

The Percent Circle is a visual aid that can help you sort out this information and solve the problem. To set up the Percent Circle, place the percent (as a decimal) in the bottom left, the starting number in the bottom right, and the ending number in the top. After you understand how to arrange the percent, the starting number, and the ending number in a Percent Circle, you have a powerful tool that can help you solve all three types of percent problems.

Each type of percent problem gives you two pieces of information and asks you to find the third. The percent circle helps you keep clear how to find this third number, either by multiplying or dividing. Just remember the following:

ߜ Multiplying the two bottom numbers always gives you the top number.

ߜ Dividing the top number by either bottom number gives you the other bottom number.

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170 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Q. Place the statement 25% of 12 is 3 into

Q. Use the Percent Circle to find 18% of 90.

the Percent Circle. Check your work by
multiplying the two bottom numbers to

A. 16.2. Place the percent (18% = 0.18) and
equal the top number.

the starting number (90) into the Percent

Circle:

A.

3

?

0 2

. 5

12

.

0 18

90

Place the percent (25% = 0.25) in the

Multiplying the two bottom numbers
bottom left, the starting number (12) in
gives you the top number:

the bottom right, and the ending number

(3) in the top. This is correct, because

0 1

. 8

0.25 · 12 = 3.

× 90

Notice that in this example, I convert

16 20

.

25% to a decimal before placing it in the

Percent Circle. Now, the top number
divided by either bottom number equals
the other bottom number:

3 ÷ 0.25 = 12

3 ÷ 12 = 0.25

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Chapter 9: Playing the Percentages 171

17. Place the statement 20% of 350 is 70 in the

18. Place the statement 17% of 150 is 25.5 in

Percent Circle. Check your work by multi-
the Percent Circle. Check your work by
plying the two bottom numbers to equal
multiplying the two bottom numbers to
the top number.

equal the top number.

Solve It

Solve It

19. Use the Percent Circle to find 79% of 11.

20. 30% of what number is 10?

Solve It

Solve It

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172 Part II: Slicing Things Up: Fractions, Decimals, and Percents

Solutions to Playing the Percentages

The following are the answers to the practice questions presented in this chapter.

a 0.9. Replace the percent sign with a decimal point and then move this decimal point two places to the left:

90% = 0.90

At the end, drop the trailing zero to get 0.9.

b 0.04. Replace the percent sign with a decimal point and then move this decimal point two places to the left:

4% = 0.04
c 0.9944. Drop the percent sign and move the decimal point two places to the left: 99.44% = 0.9944
d 2.431. Drop the percent sign and move the decimal point two places to the left: 243.1% = 2.431
e 57%. Move the decimal point two places to the right and attach a percent sign: 0.57 = 057%

At the end, drop the leading zero to get 57%.

f 30%. Move the decimal point two places to the right and attach a percent sign: 0.3 = 030%

At the end, drop the leading zero to get 30%.

g 1.5%. Move the decimal point two places to the right and attach a percent sign: 0.015 = 01.5%

At the end, drop the leading zero to get 1.5%.

h 222.2%. Move the decimal point two places to the right and attach a percent sign: 2.222 = 222.2%
i 19⁄100. Place 19 in the numerator and 100 in the denominator.

j 2⁄25. Place 8 in the numerator and 100 in the denominator:

8⁄100

You can reduce this fraction by 2 twice:

= 4⁄50 = 2⁄25
k 123⁄100. Place 123 in the numerator and 100 in the denominator: 123⁄100

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Chapter 9: Playing the Percentages 173

You can change this improper fraction to a mixed number:

= 1 23⁄100
l 33⁄4. Place 375 in the numerator and 100 in the denominator: 375⁄100

Change the improper fraction to a mixed number:

= 3 75⁄100

Reduce the fractional part of this mixed number, first by 5 and then by another 5:

= 3 15⁄20 = 3 3⁄4
m 40%. First, change 2⁄5 to a decimal:

2.0 ÷ 5 = 0.4

Now change 0.4 to a percent by moving the decimal point two places to the right and adding a percent sign:

0.4 = 40%
n 15%. First, change 3⁄20 to a decimal:

3.00 ÷ 20 = 0.15

Then change 0.15 to a percent:

0.15 = 15%
o 87.5%. First, change 7⁄8 to a decimal:

7.000 ÷ 8 = 0.875

Now change 0.875 to a percent:

0.875 = 87.5%
p 1818

.

%. First, change 2⁄11 to a decimal:

)0 1818

.

11 2 0000

.

−11

90

88

−

20

11

−

90

88

−

2

The result is the repeating decimal 0 1

. 8 . Now change this repeating decimal to a percent:

0 1

. 8 = 18 18

.

%.

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174 Part II: Slicing Things Up: Fractions, Decimals, and Percents
q

70

0 2

.

350

Place the percent (20% = 0.2) in the bottom left, the starting number (350) in the bottom right, and the ending number (70) in the top. This is correct, because 350 · 0.2 = 70.0.

r

25 5

.

0 1

. 7

150

Place the percent (17% = 0.17) in the bottom left, the starting number (150) in the bottom right, and the ending number (25.5) in the top. This is correct, because 150 · 0.17 = 25.50.

s 79% of 11 is 8.69.

?

.

0 79

11

To find the answer, multiply 0.79 · 11 to get 8.69.

t 30% of 33 3. is 10.

10

0 3

.

?

To find the answer, divide 10 by 0.3. (To divide by 0.3, begin by moving all the decimal points one place to the right, as I show you in Chapter 8).

33 3

.

3)100 0

.

9

−

10

9

−

10

9

−

1

The answer is the repeating decimal 33 3

. .

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Part III

A Giant Step Forward:

Intermediate Topics

15_288177-pp03.qxp 7/28/08 1:04 PM Page 176

In this part . . .

This part takes you into some important intermediate
math topics. I show you how scientific notation uses
decimals and powers of ten to make very large and very
small numbers easier to work with. You practice working
with both the English system and the metric system of
measurement. Then you see how to describe and measure
a variety of geometric shapes and solids. Finally, you get
some practice interpreting and working with four different
types of graphs.

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Chapter 10

Seeking a Higher Power
through Scientific Notation

In This Chapter

ᮣ Understanding powers of ten

ᮣ Multiplying and dividing powers of ten

ᮣ Converting numbers into and out of scientific notation

ᮣ Multiplying and dividing in scientific notation

P owers of ten— the number 10 multiplied by itself any number of times — form the basis of the Hindu-Arabic number system (the decimal number system) that you’re familiar with. In Chapter 1, you discover how this system uses zeros as placeholders, giving you the ones place, the tens place, the ten-trillions place, and the like. This system works well for relatively small numbers, but as numbers grow, using zeros becomes awkward. For example, ten quintillion is represented as the number 10,000,000,000,000,000,000.

Similarly, in Chapter 8, you find zeros also work as placeholders in decimals. In this case, the system works great for decimals that aren’t overly precise, but it becomes awkward when you need a high level of precision. For example, two trillionths is represented as the decimal 0.000000000002.

And really, people are busy, so who has time to write out all those placeholding zeros when they could be birdwatching, baking a pie, developing a shark-robot security system, or something else that’s more fun? Well, now you can skip some of those zeros and devote your time to more important pursuits. In this chapter, you discover scientific notation as a handy alternative way of writing very large numbers and very small decimals. Not surprisingly, scientific notation is most commonly used in the sciences, where big numbers and precise decimals show up all the time.

On the Count of Zero: Understanding

Powers of Ten

As you discover in Chapter 2, raising a number to a power multiplies the number in the base (the bottom number) by itself as many times as indicated by the exponent (the top number).

For example, 23 = 2 · 2 · 2 = 8.

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178 Part III: A Giant Step Forward: Intermediate Topics

Powers often take a long time to calculate because the numbers grow so quickly. For example, 76 may look small, but it equals 117,649. But the easiest powers to calculate are powers with a base of 10 — called, naturally, the powers of ten. You can write every power of ten in two ways:

ߜ Standard notation: As a number, such as 100,000,000

ߜ Exponential notation: As the number 10 raised to a power, such as 108

Powers of ten are easy to spot, because in standard notation, every power of ten is simply the digit 1 followed by all 0s. To raise 10 to the power of any number, just write a 1 with that number of 0s after it. For example,

100 = 1

1 with no 0s

101 = 10

1 with one 0

102 = 100

1 with two 0s

103 = 1,000

1 with three 0s

To switch from standard to exponential notation, you simply count the zeros and use that as the exponent on the number 10.

You can also raise 10 to the power of a negative number. The result of this operation is always a decimal, with the 0s coming before the 1. For example,

10–1 = 0.1

1 with one 0

10–2 = 0.01

1 with two 0s

10–3 = 0.001

1 with three 0s

10–4 = 0.0001

1 with four 0s

When expressing a negative power of ten in standard form, always count the leading 0 — that is, the 0 to the left of the decimal point. For example, 10–3 = 0.001 has three 0s, counting the leading 0.

Q. Write 103 in standard notation.

Q. Write 10–5 in standard notation.

A. 1,000. The exponent is 3, so the standard

A. 0.00001. The exponent is –5, so the stan-
notation is a 1 with three 0s after it.

dard notation is a decimal with five 0s

(including the leading 0) followed by a 1.

Q. Write 100,000 in exponential notation.

Q. Write 0.0000001 in exponential notation.

A. 105. The number 100,000 has five 0s, so
the exponential notation has 5 in the

A. 10–7. The decimal has seven 0s (including
exponent.

the leading 0), so the exponential nota-
tion has –7 in the exponent.

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Chapter 10: Seeking a Higher Power through Scientific Notation 179

1. Write each of the following powers of ten in

2. Write each of the following powers of ten in
standard notation:

exponential notation:

a. 104
a. 1,000,000,000
b. 107
b. 1,000,000,000,000
c. 1014
c. 10,000,000,000,000,000
d. 1022
d. 100,000,000,000,000,000,000,000,000,000,000

Solve It

Solve It

3. Write each of the following powers of ten in

4. Write each of the following powers of ten in
standard notation:

exponential notation:

a. 10–1
a. 0.01
b. 10–5
b. 0.000001
c. 10–11
c. 0.000000000001
d. 10–16
d. 0.000000000000000001

Solve It

Solve It

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180 Part III: A Giant Step Forward: Intermediate Topics

Exponential Arithmetic: Multiplying
and Dividing Powers of Ten

Multiplying and dividing powers of ten in exponential notation is a snap because you don’t have to do any multiplying or dividing at all — it’s nothing more than simple addition and subtraction:

ߜ Multiplication: To multiply two powers of ten in exponential notation, find the sum of the numbers’ exponents; then write a power of ten using that sum as the exponent.

ߜ Division: To divide one power of ten by another, subtract the second exponent from the first; then write a power of ten using this resulting sum as the exponent.

This rule works equally well when one or both exponents are negative — just use the rules for adding negative numbers, which I discuss in Chapter 3.

Q. Multiply 107 by 104.

Q. Find 109 ÷ 106.

A. 1011. Add the exponents 7 + 4 = 11, and

A. 103. For division, you subtract. Subtract
use this as the exponent of your answer:

the exponents 9 – 6 = 3 and use this as
the exponent of your answer:

107 · 104 = 107 + 4 = 1011

109 ÷ 106 = 10(9 – 6) = 103

5. Multiply each of the following powers

6. Divide each of the following powers of ten:

of ten:

a. 106 ÷ 104
a. 109 · 102
b. 1012 ÷ 101
b. 105 · 105
c. 10–7 ÷ 10–7
c. 1013 · 10–16
d. 1018 ÷ 100
d. 10100· 1021
e. 10100 ÷ 10–19
e. 10–15· 100
f. 10–50 ÷ 1050
f. 10–10· 10–10

Solve It

Solve It

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Chapter 10: Seeking a Higher Power through Scientific Notation 181

Representing Numbers in Scientific Notation

Numbers with a lot of zeros are awkward to work with, and making mistakes with them is easy. Scientific notation is a clearer, alternative way of representing large and small numbers.

Every number can be represented in scientific notation as the product of two numbers (two numbers multiplied together):

ߜ A decimal greater than or equal to 1 and less than 10

ߜ A power of ten written in exponential form

Use the following steps to write any number in scientific notation:

1. Write the number as a decimal (if it isn’t one already) by attaching a decimal point and one trailing zero.

2. Move the decimal point just enough places to change this decimal to a new decimal that’s greater than or equal to 1 and less than 10.

One nonzero digit should come before the decimal point.

3. Multiply the new decimal by 10 raised to the power that equals the number of places you moved the decimal point in Step 2.

4. If you moved the decimal point to the left in Step 2, the exponent is positive. If you moved it to the right (your original number was less than 1), put a minus sign on the exponent.

Q. Change the number 70,000 to scientific

Q. Change the decimal 0.000000439 to scien-
notation.

tific notation.

A. 7.0 · 104. First, write the number as a

A. 4.39 · 10–7. You’re starting with a deci-
decimal:

mal, so Step 1 — writing the number as a
decimal — is already taken care of:

70,000.0

0.000000439

Move the decimal point just enough
places to change this decimal to a new

To change 0.000000439 to a decimal
decimal that’s between 1 and 10. In this
that’s between 1 and 10, move the deci-
case, move the decimal four places to the
mal point seven places to the right and
left. You can drop all but one trailing zero:

drop the leading zeros:

7.0

4.39

You moved the decimal point four places,

Because you moved seven places to the
so multiply the new number by 104:

right, multiply the new number by 10–7:

7.0 · 104

4.39 · 10–7

Because you moved the decimal point to
the left (you started with a big number),
the exponent is a positive number, so
you’re done.

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182 Part III: A Giant Step Forward: Intermediate Topics

7. Change 2,591 to scientific notation.

8. Write the decimal 0.087 in scientific
notation.

Solve It

Solve It

9. Write 1.00000783 in scientific notation.

10. Convert 20,002.00002 to scientific notation.

Solve It

Solve It

Multiplying and Dividing with

Scientific Notation

Because scientific notation keeps track of placeholding zeros for you, multiplying and dividing by scientific notation is really a lot easier than working with big numbers and tiny decimals that have tons of zeros.

To multiply two numbers in scientific notation, follow these steps:

1. Multiply the two decimal parts to find the decimal part of the answer.

See Chapter 8 for info on multiplying decimals.

2. Add the exponents on the 10s to find the power of ten in the answer.

You’re simply multiplying the powers of ten, as I show you earlier in “Exponential Arithmetic: Multiplying and Dividing Powers of Ten.”

3. If the decimal part of the result is 10 or greater, adjust the result by moving the decimal point one place to the left and adding 1 to the exponent.

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Chapter 10: Seeking a Higher Power through Scientific Notation 183

Here’s how to divide two numbers in scientific notation:

1. Divide the decimal part of the first number by the decimal part of the second number to find the decimal part of the answer.

2. To find the power of ten in the answer, subtract the exponent on the second power of ten from the exponent on the first.

You’re really just dividing the first power of ten by the second.

3. If the decimal part of the result is less than 1, adjust the result by moving the decimal point one place to the right and subtracting 1 from the exponent.

Q. Multiply 2.0 · 103 by 4.1 · 104.

Q. Divide 3.4 · 104 by 2.0 · 109.

A. 8.2 · 107. Multiply the two decimal

A. 1.7 · 10–5. Divide the first decimal part by
parts:

the second:

2.0 · 4.1 = 8.2

3.4 ÷ 2.0 = 1.7

Then multiply the powers of ten by

Then divide the first power of ten by the
adding the exponents:

second by subtracting the exponents:

103 · 104 = 103 + 4 = 107

104 ÷ 109 = 104 – 9 = 10–5

In this case, no adjustment is neces-

In this case, no adjustment is necessary
sary because the resulting decimal
because the resulting decimal part isn’t
part is less than 10.

less than 1.

11. Multiply 1.5 · 107 by 6.0 · 105.

12. Divide 6.6 · 108 by 1.1 · 103.

Solve It

Solve It

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184 Part III: A Giant Step Forward: Intermediate Topics

Answers to Problems in Seeking a Higher

Power through Scientific Notation

The following are the answers to the practice questions presented in this chapter.

a In each case, write the digit 1 followed by the number of 0s indicated by the exponent: a. 104 = 10,000
b. 107 = 10,000,000
c. 1014 = 100,000,000,000,000
d. 1022 = 10,000,000,000,000,000,000,000
b In each case, count the number of 0s; then write a power of ten with this number as the exponent.

a. 1,000,000,000 = 109
b. 1,000,000,000,000 =1012
c. 10,000,000,000,000,000 = 1016
d. 100,000,000,000,000,000,000,000,000,000,000 = 1032
c Write a decimal beginning with all 0s and ending in 1. The exponent indicates the number of 0s in this decimal (including the leading 0):

a. 10–1 = 0.1
b. 10–5 = 0.00001
c. 10–11 = 0.00000000001
d. 10–16 = 0.0000000000000001
d In each case, count the number of 0s (including the leading 0); then write a power of ten using this number negated (with a minus sign) as the exponent:

a. 0.01 = 10–2
b. 0.000001 = 10–6
c. 0.000000000001 = 10–12
d. 0.000000000000000001 = 10–18
e Add the exponents and use this sum as the exponent of the answer.

a. 109 · 102 = 109 + 2 = 1011
b. 105 · 105 = 105 + 5 = 1010
c. 1013 · 10–16 = 1013 + –16 = 10–3
d. 10100· 1021 = 10100 + 21 = 10121
e. 10–15· 100 = 10–15 + 0 = 10–15
f. 10–10· 10–10 = 10–10 + –10 = 10–20

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Chapter 10: Seeking a Higher Power through Scientific Notation 185
f In each case, subtract the first exponent minus the second and use this result as the exponent of the answer.

a. 106 ÷ 104 = 106 – 4 = 102
b. 1012 ÷ 101 = 1012 – 1 = 1011
c. 10–7 ÷ 10–7 = 10–7 – (–7) = 10–7 + 7 = 100
d. 1018 ÷ 100 = 1018 – 0 = 1018
e. 10100 ÷ 10–19 = 10100 – (–19) = 10100 + 19 = 10119
f. 10–50 ÷ 1050 = 10–50 – 50 = 10–100
g 2,591= 2.591 · 103. Write 2,591 as a decimal:

2,591.0

To change 2,591.0 to a decimal between 1 and 10, move the decimal point three places to the left and drop the trailing zero:

2.591

Because you moved the decimal point three places, multiply the new decimal by 103: 2.591 · 103

You moved the decimal point to the left, so the exponent stays positive. The answer is 2.591 · 103.

h 0.087 = 8.7 · 10–2. To change 0.087 to a decimal between 1 and 10, move the decimal point two places to the right and drop the leading zero:

8.7

Because you moved the decimal point two places, multiply the new decimal by 102. You moved the decimal point to the right, so put a minus sign on the exponent:

8.7 · 10–2
i 1.00000783 = 1.00000783 · 100. The decimal 1.00000783 is already between 1 and 10, so you don’t have to move the decimal point. Because you move the decimal point zero places, multiply the new decimal by 100:

1.00000783 · 100
j 20,002.00002 = 2.0002000002 · 104. The number 20,002.00002 is already a decimal. To change it to a decimal between 1 and 10, move the decimal point four places to the left: 2.000200002

Because you moved the decimal point four places, multiply the new decimal by 104: 2.000200002 · 104

You moved the decimal point to the left, so the answer is 2.000200002 · 104.

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186 Part III: A Giant Step Forward: Intermediate Topics
k (1.5 · 107 )(6.0 · 105) = 9.0 · 1012. Multiply the two decimal parts: 1.5 · 6.0 = 9.0

Multiply the two powers of ten:

107 · 105 = 107 + 5 = 1012

In this case, no adjustment is necessary because the decimal is less than 10.

l (6.6 · 108) ÷ (1.1 · 103) = 6.0 · 105. Divide the first decimal part by the second: 6.6 ÷ 1.1 = 6.0

Divide the first power of ten by the second:

108 ÷ 103 = 108 – 3 = 105

In this case, no adjustment is necessary because the decimal is greater than 1.

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Chapter 11

Weighty Questions on

Weights and Measures

In This Chapter

ᮣ Reviewing the English system of measurement

ᮣ Understanding the metric system of measurement

ᮣ Knowing a few tricks for approximating metric units in English units

ᮣ Converting more precisely between metric and English units

Units connect numbers to the real world. For example, the number 2 is just an idea until you attach a unit: 2 apples, 2 children, 2 houses, 2 giant squid, and so forth. Apples, children, houses, and giant squid are easy to do math with because they’re all discrete —
that is, they’re separate and easy to count one by one. For example, if you’re working with a basket of apples, applying the Big Four operations is quite straightforward. You can add a few apples to the basket, divide them into separate piles, or perform any other operation that you like.

However, lots of things aren’t discrete but rather continuous — that is, they’re difficult to separate and count one by one. To measure the length of a road, the amount of water in a bucket, the weight of a child, the amount of time a job takes to do, or the temperature of Mount Erebus (and other Antarctic volcanoes), you need units of measurement.

The two most common systems of measurement are the English system (used in the United States) and the metric system (used throughout the rest of the world). In this chapter, I famil-iarize you with both systems. Then I show you how to make conversions between both systems.

The Basics of the English System

The English system of measurement is most commonly used in the United States. If you were raised in the States, you’re probably familiar with most of these units. Table 11-1 shows you the most common units and some equations so you can do simple conversions from one unit to another.

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188 Part III: A Giant Step Forward: Intermediate Topics

Table 11-1

Commonly Used English Units of Measurement

Measure of

English Units

Conversion Equations

Distance (length)

Inches (in.)

Feet (ft.)

12 inches = 1 foot

Yards (yd.)

3 feet = 1 yard

Miles (mi.)

5,280 feet = 1 mile

Fluid volume

Fluid ounces (fl. oz.)

(capacity)

Cups (c.)

8 fluid ounces = 1 cup

Pints (pt.)

2 cups = 1 pint

Quarts (qt.)

2 pints = 1 quart

Gallons (gal.)

4 quarts = 1 gallon

Weight

Ounces (oz.)

Pounds (lb.)

16 ounces = 1 pound

Tons

2,000 pounds = 1 ton

Time

Seconds

Minutes

60 seconds = 1 minute

Hours

60 minutes = 1 hour

Days

24 hours = 1 day

Weeks

7 days = 1 week

Years

365 days = 1 year

Temperature

Degrees Fahrenheit (°F)

Speed (rate)

Miles per hour (mph)

To use this chart, remember the following rules:

ߜ When converting from a large unit to a smaller unit, always multiply. For example, 2 pints are in 1 quart, so to convert 10 quarts to pints, multiply by 2:

10 quarts · 2 = 20 pints

ߜ When converting from a small unit to a larger unit, always divide. For example, 3 feet are in 1 yard, so to convert 12 feet to yards, divide by 3:

12 feet ÷ 3 = 4 yards

When converting from large units to very small ones (for example, from tons to ounces), you may need to multiply more than once. Similarly, when converting from small units to much larger ones (for example, from minutes to days), you may need to divide more than once.

After doing a conversion, step back and apply a reasonability test to your answer — that is, think about whether your answer makes sense. For example, when you convert feet to inches, the number you end up with should be a lot bigger than the number you started with because there are lots of inches in a foot.

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Chapter 11: Weighty Questions on Weights and Measures 189

Q. How many minutes are in a day?

Q. If you have 32 fluid ounces, how many
pints do you have?

A. 1,440 minutes. You’re going from a larger
unit (a day) to a smaller unit (minutes);

A. 2 pints. 8 fluid ounces are in a cup, so

60 minutes are in an hour and 24 hours
divide as follows:

are in a day, so you simply multiply:

32 fluid ounces ÷ 8 = 4 cups

60 · 24 = 1,440 minutes

And 2 cups are in a pint, so divide again:

Therefore, 1,440 minutes are in a day.

4 cups ÷ 2 = 2 pints

Q. 5 pints = ________________ fluid ounces.

Therefore, 32 fluid ounces equals 2 pints.

A. 80 fluid ounces. 8 fluid ounces are in a

Q. 504 hours = ________________ weeks.

cup and 2 cups are in a pint, so

8 · 2 = 16 fl. oz.

A. 3 weeks. 24 hours are in a day, so divide
as follows:

16 fluid ounces are in a pint, so

504 hours ÷ 24 = 21 days

5 pints = 5 · 16 fluid ounces = 80 fl. oz.

And 7 days are in a week, so divide again:

21 days ÷ 7 = 3 weeks

Therefore, 504 hours equals 3 weeks.

1. Answer each of the following questions:

2. Calculate each of the following:

a. How many inches are in a yard?

a. 7 quarts = ________________ cups
b. How many hours are in a week?

b. 5 miles = ________________ inches
c. How many ounces are in a ton?

c. 3 gallons = ________________ fluid
ounces
d. How many cups are in a gallon?

d. 4 days = ________________ seconds

Solve It

Solve It

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190 Part III: A Giant Step Forward: Intermediate Topics

3. Answer each of the following questions:

4. Calculate each of the following:

a. If you have 420 minutes, how many
a. 168 inches = ________________ feet
hours do you have?

b. 100 quarts = ________________ gallons
b. If you have 144 inches, how many yards
c. 288 ounces = ________________ pounds
do you have?

d. 76 cups = ________________ quarts
c. If you have 22,000 pounds, how many
tons do you have?

Solve It
d. If you have 256 fluid ounces, how many
gallons do you have?

Solve It

Going International with the Metric System

The metric system is the most commonly used system of measurement throughout the world. Scientists and others who like to keep up with the latest lingo (since the 1960s) often refer to it as the International System of Units, or SI. Unlike the English system, the metric system is based exclusively on powers of ten (see Chapter 10). This feature makes the metric system much easier to use (after you get the hang of it!) than the English system because you can do a lot of calculations simply by moving the decimal point.

The metric system includes five basic units, shown in Table 11-2.

Table 11-2

Five Basic Metric Units

Measure of

Metric Units

Distance (length)

Meters (m)

Fluid volume (capacity)

Liters (L)

Mass (weight)

Grams (g)

Time

Seconds (s)

Temperature

Degrees Celsius/Centigrade (°C)

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Chapter 11: Weighty Questions on Weights and Measures 191

You can modify each basic metric unit with the prefixes shown in Table 11-3. When you know how the metric prefixes work, you can use them to make sense even of units that you’re not familiar with.

Table 11-3

Metric Prefixes

Prefix

Meaning

Number

Power of Ten

Tera-

One trillion

1,000,000,000,000

1012

Giga-

One billion

1,000,000,000

109

Mega-

One million

1,000,000

106

Kilo-

One thousand

1,000

103

Hecta-

One hundred

100

102

Deca-

Ten

10

101

(none)

One

1

100

Deci-

One tenth

0.1

10–1

Centi-

One hundredth

0.01

10–2

Milli-

One thousandth

0.001

10–3

Micro-

One millionth

0.000001

10–6

Nano-

One billionth

0.000000001

10–9

Q. How many millimeters are in a meter?

Q. A dyne is an old unit of force, the push or
pull on an object. Using what you know

A. 1,000. The prefix milli- means one
about metric prefixes, how many dynes
thousandth, so a millimeter is 1⁄1,000 of a
do you think are in 14 teradynes?

meter. Therefore, a meter contains 1,000
millimeters.

A. 14,000,000,000,000 (14 trillion). The
prefix tera- means one trillion, so

1,000,000,000,000 dynes are in a teradyne;
therefore,

14 teradynes = 14 · 1,000,000,000,000
dynes = 14,000,000,000,000 dynes

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192 Part III: A Giant Step Forward: Intermediate Topics

5. Give the basic metric unit for each type of

6. Write down the number or decimal associ-
measurement listed below:

ated with each of the following metric
prefixes:

a. The amount of vegetable oil for a recipe
a. kilo-
b. The weight of an elephant
b. milli-
c. How much water a swimming pool
can hold
c. centi-
d. How hot a swimming pool is
d. mega-
e. How long you can hold your breath
e. micro-
f. Your height
f. giga-
g. Your weight
g. nano-
h. How far you can run
h. no prefix

Solve It

Solve It

7. Answer each of the following questions:

8. Using what you know about metric pre-
fixes, calculate each of the following:

a. How many centimeters are in a meter?

a. 75 kilowatts = ________________ watts
b. How many milliliters are in a liter?

b. 12 seconds = ________________
c. How many milligrams are in a kilogram?

microseconds
d. How many centimeters are in a kilometer?

c. 7 megatons = ________________ tons

Solve It
d. 400 gigaHertz = ________________ Hertz

Solve It

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Chapter 11: Weighty Questions on Weights and Measures 193

Converting Between English and Metric Units

To convert between metric units and English units, use the four conversion equations shown in the first column of Table 11-4.

Table 11-4

Conversion Factors for English and Metric Units

Conversion Equation

English-to-Metric

Metric-to-English

1 m

3 2

. 6 ft.

1 meter ≈ 3.26 feet

3 2

. 6 ft

1 m

1 km

0 6

. 2 mi.

1 kilometer ≈ 0.62 miles

0 6

. 2 mi.

1 km

1 L

0 2

. 6 gal.

1 liter ≈ 0.26 gallons

0 2

. 6 gal.

1 L

1 kg

2 2

. 0 lb.

1 kilogram ≈ 2.20 pounds

2 2

. 0 lb.

1 kg

The remaining columns in Table 11-6 show conversion factors (fractions) that you multiply by to convert from metric units to English or from English units to metric. To convert from one unit to another, multiply by the conversion factor and cancel out any unit that appears in both the numerator and denominator.

Always use the conversion factor that has units you’re converting from in the denominator.

For example, to convert from miles to kilometers, use the conversion factor that has miles in 1 km
the denominator: that is,

.

0 6

. 2 mi.

Sometimes, you may want to convert between units for which no direct conversion factor exists. In these cases, set up a conversion chain to convert via one or more intermediate units. For instance, to convert centimeters into inches, you may go from centimeters to meters to feet to inches. When the conversion chain is set up correctly, every unit cancels out except for the unit that you’re converting to. You can set up a conversion chain of any length to solve a problem.

With a long conversion chain, it’s sometimes helpful to take an extra step and turn the whole chain into a single fraction. Place all the numerators above a single fraction bar and all the denominators below it, keeping multiplication signs between the numbers.

A chain can include conversion factors built from any conversion equation in this chapter.

For example, you know from “The Basics of the English System” that 2 pints = 1 quart, so you can use the following two fractions:

2 pt.

1 qt.

1 qt.

2 pt.

Similarly, you know from “Going International with the Metric System” that 1 kilogram = 1,000
grams, so you can use these two fractions:

1 kg

1,000 g

1,000 g

1 kg

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194 Part III: A Giant Step Forward: Intermediate Topics

Q. Convert 5 kilometers to miles.

fraction with kilograms in the numerator
and grams in the denominator:

A. 3.1 miles. To convert from kilometers,

1 kg
multiply 5 kilometers by the conversion

21 g ·
factor with kilometers in the denominator:

1,000 g

0 6

. 2 mi.

To convert kilograms to pounds, use the

5 km · 1 km
equation 1 kg = 2.2 lb and multiply by the
fraction with pounds in the numerator

Now you can cancel the unit kilometer in
and kilograms in the denominator:

both the numerator and denominator:

1 kg

2.2 lb.

= 21 g ·

·

0 6

. 2 mi.

1 kg

= 5 km ·

1,000 g

1 km

Cancel grams and kilograms in both the

Calculate the result:

numerator and denominator:

0 6

. 2 mi.

1 kg

2.2 lb.

= 5 ·

= 3 1

. mi.

= 21 g · ·

1

1,000 g

1 kg

Notice that when you set up the conver-

Finally, calculate the result:

sion correctly, you don’t have to think

1

2.2 lb.

about the unit — it changes from kilo-

= 21 ·

·

=

1,000

1
meters to miles automatically!

0.021 · 2.2 lbs = 0.0462 lb.

Q. Convert 21 grams to pounds.

A.

After doing the conversion, step back

0.0462 pounds. You don’t have a conver-
and apply a reasonability test to your
sion factor to convert grams to pounds
answers. Does each answer make sense?

directly, so set up a conversion chain

For example, when you convert grams to
that makes the following conversion:

kilograms, the number you end up with
grams → kilograms → pounds
should be a lot smaller than the number
you started with because each kilogram

To convert grams to kilograms, use the
contains lots of grams.

equation 1,000 g = 1 kg. Multiply by the

9. Convert 8 kilometers to miles.

10. If you weigh 72 kilograms, what’s your
weight in pounds to the nearest whole

Solve It
pound?

Solve It

11. If you’re 1.8 meters tall, what’s your height

12. Change 100 cups to liters, rounded to the
in inches to the nearest whole inch?

nearest whole liter.

Solve It

Solve It

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Chapter 11: Weighty Questions on Weights and Measures 195

Answers to Problems in Weighty Questions
on Weights and Measures

The following are the answers to the practice questions presented in this chapter.

a All these questions ask you to convert a large unit to a smaller one, so use multiplication.

a. How many inches are in a yard? 36 inches. 12 inches are in a foot and 3 feet are in a yard, so 3 · 12 = 36 in.

b. How many hours are in a week? 168 hours. 24 hours are in a day and 7 days are in a week, so 24 · 7 = 168 hours
c. How many ounces are in a ton? 32,000 ounces. 16 ounces are in a pound and 2,000 pounds are in a ton, so

16 · 2,000 = 32,000 oz.

d. How many cups are in a gallon? 16 cups. 2 cups are in a pint, 2 pints are in a quart, and 4
quarts are in a gallon, so

2 · 2 · 4 = 16 c.

b All these problems ask you to convert a large unit to a smaller one, so use multiplication: a. 7 quarts = 28 cups. 2 cups are in a pint and 2 pints are in a quart, so 2 · 2 = 4 c.

So 4 cups are in a quart; therefore,

7 qt. = 7 · 4 c. = 28 c.

b. 5 miles = 316, 800 inches. 12 inches are in a foot and 5,280 feet are in a mile, so 12 · 5,280 = 63,360 in.

63,360 inches are in a mile, so

5 mi. = 5 · 63,360 in. = 316,800 in.

c. 3 gallons = 384 fluid ounces. 8 fluid ounces are in a cup, 2 cups are in a pint, 2 pints are in a quart, and 4 quarts are in a gallon, so

8 · 2 · 2 · 4 = 128 fl. oz.

Therefore, 128 fluid ounces are in a gallon, so

3 gal. = 3 · 128 fl. oz. = 384 fl. oz.

d. 4 days = 345,600 seconds. 60 seconds are in a minute, 60 minutes are in an hour, and 24 hours are in a day, so

60 · 60 · 24 = 86,400 seconds

86,400 seconds are in a day, so

4 days = 4 · 86,400 seconds = 345,600 seconds

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196 Part III: A Giant Step Forward: Intermediate Topics
c All these problems ask you to convert a smaller unit to a larger one, so use division: a. If you have 420 minutes, how many hours do you have? 7 hours. There are 60 minutes in an hour, so divide by 60:

420 minutes ÷ 60 = 7 hours
b. If you have 144 inches, how many yards do you have? 4 yards. There are 12 inches in a foot, so divide by 12:

144 in. ÷ 12 = 12 ft.

There are 3 feet in a yard, so divide by 3:

12 ft. ÷ 3 = 4 yd.

c. If you have 22,000 pounds, how many tons do you have? 11 tons. There are 2,000 pounds in a ton, so divide by 2,000

22,000 lb. ÷ 2,000 = 11 tons
d. If you have 256 fluid ounces, how many gallons do you have? 2 gallons. There are 8 fluid ounces in a cup, so divide by 8:

256 fl. oz. ÷ 8 = 32 c.

There are 2 cups in a pint, so divide by 2:

32 c. ÷ 2 = 16 pt.

There are 2 pints in a quart, so divide by 2:

16 pt. ÷ 2 = 8 qt.

There are 4 quarts in a gallon, so divide by 4:

8 qt. ÷ 4 = 2 gal.

d All these problems ask you to convert a smaller unit to a larger one, so use division: a. 168 inches = 14 feet. There are 12 inches in a foot, so divide by 12: 168 in. ÷ 12 = 14 ft.

b. 100 quarts = 25 gallons. There are 4 quarts in a gallon, so divide by 4: 100 qt. ÷ 4 = 25 gal.

c. 288 ounces = 18 pounds. There are 16 ounces in a pound, so divide by 16: 288 oz. ÷ 16 = 18 lbs.

d. 76 cups = 19 quarts. There are 2 cups in a pint, so divide by 2: 76 cups ÷ 2 = 38 pints

There are 2 pints in a quart, so divide by 2:

38 pints ÷ 2 = 19 quarts
e Give the basic metric unit for each type of measurement listed below. Note that you want only the base unit, without any prefixes.

a. The amount of vegetable oil for a recipe: liters
b. The weight of an elephant: grams

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Chapter 11: Weighty Questions on Weights and Measures 197
c. How much water a swimming pool can hold: liters
d. How hot a swimming pool is: degrees Celsius (Centigrade)

e. How long you can hold your breath: seconds
f. Your height: meters
g. Your weight: grams
h. How far you can run: meters
f Write down the number or decimal associated with each of the following metric prefixes: a. kilo-: 1,000 (one thousand or 103 )

b. milli-: 0.001 (one thousandth or 10–3 )

c. centi-: .01 (one hundredth or 10–2 )

d. mega-: 1,000,000 (one million or 106 )

e. micro-: 0.000001 (one millionth or 10–6 )

f. giga-: 1,000,000,000 (one billion or 109 )

g. nano-: 0.000000001 (one billionth or 10–9 )

h. no prefix: 1 (one or 100 )

g Answer each of the following questions:

a. How many centimeters are in a meter? 100 centimeters
b. How many milliliters are in a liter? 1,000 milliliters
c. How many milligrams are in a kilogram? 1,000,000 milligrams. 1,000 milligrams are in a gram and 1,000 grams are in a kilogram, so

1,000 · 1,000 = 1,000,000 mg

Therefore, 1,000,000 milligrams are in a kilogram.

d. How many centimeters are in a kilometer? 100,000 centimeters. 100 centimeters are in a meter and 1,000 meters are in a kilometer, so

100 · 1,000 = 100,000 cm

Therefore, 100,000 centimeters are in a kilometer.

h Using what you know about metric prefixes, calculate each of the following: a. 75 kilowatts = 75,000 watts. The prefix kilo- means one thousand, so 1,000 watts are in a kilowatt; therefore,

75 kilowatts = 75 · 1,000 watts = 75,000 watts
b. 12 seconds = 12,000,000 microseconds. The prefix micro- means one millionth, so a microsecond is a millionth of a second. Therefore, 1,000,000 microseconds are in a second. Thus, 12 seconds = 12 · 1,000,000 microseconds = 12,000,000 microseconds
c. 7 megatons = 7,000,000 tons. The prefix mega- means one million, so 1,000,000 tons are in a megaton; therefore,

7 megatons = 7 · 1,000,000 tons = 7,000,000 tons

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198 Part III: A Giant Step Forward: Intermediate Topics
d. 400 gigaHertz = 400,000,000,000 Hertz. The prefix giga- means one billion, so 1,000,000,000

Hertz are in a gigaHertz; thus,

400 gigaHertz = 400 · 1,000,000,000 Hertz = 400,000,000,000 gigaHertz
i 8 kilometers = 4.96 miles. To convert kilometers to miles, multiply by the conversion fraction with miles in the numerator and kilometers in the denominator:

8 km · 0 6

. 2 mi.

1 km

Cancel the unit kilometer in both the numerator and denominator:

= 8 km · 0 6

. 2 mi.

1 km

Now calculate the result:

= 8 · 0.62 mi. = 4.96 mi.

j To the nearest pound, 72 kilograms = 158 pounds. To convert kilograms to pounds, multiply by the conversion factor with pounds in the numerator and kilograms in the denominator: 2.2 lb.

72 kg · 1 kg

Cancel the unit kilogram in both the numerator and denominator:

2.2 lb.

= 72 kg · 1 kg

= 72 · 2.2 lb.

Multiply to find the answer:

= 158.4 lb.

Round to the nearest whole pound:

≈ 158 lb.

k To the nearest inch, 1.8 meters = 70 inches. You don’t have a conversion factor to change meters to inches directly, so set up a conversion chain as follows:

meters → feet → inches

Convert meters to feet with the fraction that puts meters in the denominator: 3 2

. 6 ft.

1.8 m · 1 m

Convert the feet to inches with the conversion factor that has feet in the denominator: 3 2

. 6 ft.

12 in.

= 1.8 m ·

·

1 m

1 ft.

Cancel the units meters and feet in both the numerator and denominator: 3 2

. 6 ft.

12 in.

= 1.8 m · ·

1 m

1 ft.

3 2

. 6

12 in.

= 1.8 ·

·

1

1

= 1.8 · 3.26 · 12 in.

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Chapter 11: Weighty Questions on Weights and Measures 199

Multiply to find the answer:

= 70.416 in.

Round the answer to the nearest whole inch:

≈ 70 in.

l To the nearest liter, 100 cups = 24 liters. You don’t have a conversion factor to change cups to liters directly, so set up a conversion chain:

cups → pints → quarts → gallons → liters

Convert cups to pints:

1 pt.

100 c. · 2 c.

Convert pints to quarts:

1 pt.

1 qt.

= 100 c. ·

·

2 c.

2 pt.

Convert quarts to gallons:

1 pt.

1 qt.

1 gal.

= 100 c. ·

·

·

2 c.

2 pt.

4 qt.

Convert gallons to liters:

1 pt.

1 qt.

1 gal.

1 L

= 100 c. ·

·

·

·

2 c.

2 pt.

4 qt.

0.26 gal.

Now all units except liters cancel out:

1 pt.

1 qt.

1 gal.

1 L

= 100 c. · · · ·

2 c.

2 pt.

4 qt.

0.26 gal.

1

1

1

1 L

=

100

· · · ·

2

2

4

0 2

. 6

To avoid confusion, set this chain up as a single fraction:

100 ⋅1⋅1⋅1⋅1 L

2 ⋅ 2 ⋅ 4 ⋅ 0 2

. 6

100 L

= 4 1.6

Use decimal division to find the answer to at least one decimal place:

≈ 24.04 L

Round to the nearest whole liter:

≈ 24 L

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200 Part III: A Giant Step Forward: Intermediate Topics

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Chapter 12

Shaping Up with Geometry

In This Chapter

ᮣ Finding the area of a triangle

ᮣ Using the Pythagorean theorem

ᮣ Measuring quadrilaterals

ᮣ Finding the area and circumference of a circle

ᮣ Calculating the volume of a variety of solids

G eometry is the study of shapes and figures, and it’s really popular with the Ancient Greeks, architects, engineers, carpenters, robot designers, and high school math teachers.

A shape is any closed two-dimensional (2-D) geometric figure that has an inside and an outside, and a solid is just like a shape, only it’s three-dimensional (in 3-D). In this chapter, you work with three important shapes: triangles, quadrilaterals, and circles. I show you how to find the area and in some cases the perimeter (the length of the edge) of these shapes. I also focus on a variety of solids, showing you how to find the volume of each.

Getting in Shape: Polygon (And

Nonpolygon) Basics

You can divide shapes into two basic types: polygons and nonpolygons. A polygon has all straight sides, and you can easily classify polygons by the number of sides they have:

Polygon

Number of Sides

Triangle

3

Quadrilateral

4

Pentagon

5

Hexagon

6

Heptagon

7

Octagon

8

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202 Part III: A Giant Step Forward: Intermediate Topics

Any shape that has at least one curved edge is a nonpolygon. The most common nonpolygon is the circle.

The area of a shape — the space inside — is usually measured in square units, such as square inches (in.2), square meters (m2), or square kilometers (km2). If a problem mixes units of measurement, such as inches and feet, you have to convert to one or the other before doing the math (for more on conversions, see Chapter 11).

Making a Triple Play with Triangles

Any shape with three straight sides is a triangle. To find the area of a triangle, use the following formula, in which b is the length of the base (one side of the triangle) and h is the height of the triangle (the shortest distance from the base to the opposite corner); also see Figure 12-1: A = 1⁄2 · b · h

Triangle
height ( h)

Figure 12-1:

The area of
a triangle
using the
base (b) and
height (h).

base ( b)

Any triangle that has a 90° angle is called a right triangle. The right triangle is one of the most important shapes in geometry. In fact, trigonometry is devoted entirely to the study of right triangles.

The longest side of a right triangle (c) is called the hypotenuse, and the two short sides ( a and b) are each called legs. The most important right triangle formula — called the Pythagorean theorem — allows you to find the length of the hypotenuse given only the length of the legs: a 2 + b 2 = c 2

Figure 12-2 shows this theorem in action.

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Chapter 12: Shaping Up with Geometry 203

Right Triangle
hypotenuse (

Figure 12-2:

c)

leg ( a)

Using the

Pythagorean
theorem to
find the
hypotenuse

(c) of a right
triangle.

leg ( b)

Q. Find the area of a triangle with a base of

Q. Find the hypotenuse of a right triangle

5 meters and a height of 6 meters.

with legs that are 6 inches and 8 inches
in length.

A. 15 square meters.

A. 10 inches. Use the Pythagorean theorem

A = 1⁄2 · 5 m · 6 m = 15 m2
to find the value of c as follows:

a 2 + b 2 = c 2

62 + 82 = c 2

36 + 64 = c 2

100 = c 2

So when you multiply c by itself, the
result is 100. Therefore, c = 10 in.,
because 10 · 10 = 100.

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204 Part III: A Giant Step Forward: Intermediate Topics

1. What’s the area of a triangle with a base of

2. Find the area of a triangle with a base of 10

7 centimeters and a height of 4 centimeters?

kilometers and a height of 17 kilometers.

Solve It

Solve It

3. Figure out the area of a triangle with a base

4. Discover the hypotenuse of a right triangle
of 2 feet and a height of 33 inches.

whose two legs measure 3 miles and 4 miles.

Solve It

Solve It

5. What’s the hypotenuse of a right triangle

6. Calculate the hypotenuse of a right triangle
with two legs measuring 5 millimeters and
with two legs measuring 8 feet and 15 feet.

12 millimeters?

Solve It

Solve It

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Chapter 12: Shaping Up with Geometry 205

Taking a Fourth Side with Quadrilaterals

Any shape with four sides is a quadrilateral. Quadrilaterals include squares, rectangles, rhombuses, parallelograms, and trapezoids, plus a host of more irregular shapes. In this section, I show you how to find the area (A) and in some cases the perimeter (P) of these five basic types of quadrilaterals.

A square has four right angles and four equal sides. To find the area and perimeter of a square, use the following formulas, where s stands for the length of a side (see Figure 12-3): A = s 2

P = 4 · s

Square
side ( s)

Figure 12-3:

The area
and perim-
eter of a
square
using the
length of a
side (s).

A rectangle has four right angles and opposite sides that are equal. The long side of a rectangle is called the length, and the short side is called the width. To find the area and perimeter of a rectangle, use the following formulas, where l stands for the length of a side and w stands for width (see Figure 12-4):

A = l · w

P = 2 · ( l + w)

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206 Part III: A Giant Step Forward: Intermediate Topics

Rectangle

Figure 12-4:

The area
width ( w)

and peri-
meter of a
rectangle
using the
length (l)

and width

(w).

length ( l)

A rhombus resembles a collapsed square. It has four equal sides, but its four angles aren’t necessarily right angles. Similarly, a parallelogram resembles a collapsed rectangle. Its opposite sides are equal, but its four angles aren’t necessarily right angles. To find the area of a rhombus or a parallelogram, use the following formula, where b stands for the length of the base (either the bottom or top side) and h stands for the height (the shortest distance between the two bases); also see Figure 12-5:

A = b · h

Rhombus

Parallelogram

Figure 12-5:

The area of
a rhombus
or parallelo-
height ( h)

height ( h)

gram using
the base (b)

and
height (h).

base ( b)

base ( b)

A trapezoid is a quadrilateral whose only distinguishing feature is that it has two parallel bases (top side and bottom side). To find the area of a trapezoid, use the following formula, where b 1 and b 2 stand for the lengths of the two bases and h stands for the height (the shortest distance between the two bases); also see Figure 12-6:

A = 1⁄2 · ( b 1 + b 2) · h

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Chapter 12: Shaping Up with Geometry 207

Trapezoid
top base ( b )

1

Figure 12-6:

The area of
a trapezoid
height ( h)

using the
length of the
two bases

(b1 and b2)

and the
height (h).

bottom base ( b )

2

Q. Find the area and perimeter of a square

Q. Find the area of a parallelogram with a
with a side that measures 5 inches.

base of 4 feet and a height of 3 feet.

A. The area is 25 square inches, and the

A. The area is 12 square feet.

perimeter is 20 inches.

A = b · h = 4 ft. · 3 ft. = 12 ft.2

A = s 2 = (5 in.)2 = 25 in.2

P = 4 · s = 4 · 5 in. = 20 in.

Q. Find the area of a trapezoid with bases
of 3 meters and 5 meters and a height of

Q.

2 meters.

Find the area and perimeter of a rectan-
gle with a length of 9 centimeters and a

A. The area is 8 square meters.

width of 4 centimeters.

A = 1⁄2 · ( b 1 + b 2) · h

A. The area is 36 square centimeters, and

= 1⁄
the perimeter is 26 centimeters.

2 · (3 m + 5 m) · 2 m

= 1⁄

A = l · w = 9 cm · 4 cm = 36 cm2

2 · (8 m) · 2 m = 8 m2

P = 2 · ( l + w) = 2 · (9 cm + 4 cm) =

2 · 13 cm = 26 cm

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208 Part III: A Giant Step Forward: Intermediate Topics

7. What are the area and perimeter of a

8. Find the area and perimeter of a square
square with a side of 9 miles?

with a side of 31 centimeters.

Solve It

Solve It

9. Figure out the area and perimeter of a rec-

10. Determine the area and perimeter of a rec-
tangle with a length of 10 inches and a
tangle that has a length of 23 kilometers
width of 5 inches.

and a width of 19 kilometers.

Solve It

Solve It

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Chapter 12: Shaping Up with Geometry 209

11. What’s the area of a rhombus with a base

12. Figure out the area of a parallelogram with
of 9 meters and a height of 6 meters?

a base of 17 yards and a height of 13 yards.

Solve It

Solve It

13. Write down the area of a trapezoid with

14. What’s the area of a trapezoid that has
bases of 6 feet and 8 feet and a height of
bases of 15 millimeters and 35 millimeters

5 feet.

and a height of 21 millimeters?

Solve It

Solve It

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210 Part III: A Giant Step Forward: Intermediate Topics

Getting Around with Circle Measurements

A circle is the set of all points that are a constant distance from a point inside it. Here are a few terms that are handy when talking about circles (see Figure 12-7):

ߜ The center (c) of a circle is the point that’s the same distance from any point on the circle itself.

ߜ The radius (r) of a circle is the distance from the center to any point on the circle.

ߜ The diameter (d) of a circle is the distance from any point on the circle through the center to the opposite point on the circle.

Circle
center

Figure 12-7:

( c)

r ( d)

The area
radius ( r)

and circum-
diamete
ference of a
circle using
d = 2 r
the radius

Area = ␲ r2

(r).

Circumference= 2 ␲ r2

To find the area (A) of a circle, use the following formula:

A = π · r 2

The symbol π is called pi (pronounced pie). It’s a decimal that goes on forever, so you can’t get an exact value for π. However, the number 3.14 is a good approximation of π that you can use when solving problems that involve circles. (Note that when you use an approximation, the ≈ symbol replaces the = sign in problems.)

The perimeter of a circle has a special name: the circumference (C). The formulas for the circumference of a circle also inclues π:

C = 2 · π · r

C = π · d

These circumference formulas say the same thing because as you can see in Figure 12-7, the diameter of a circle is always twice the radius of that circle. That gives you the following formula:

d = 2 · r

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Chapter 12: Shaping Up with Geometry 211

Q. What’s the diameter of a circle that has a

Q. What’s the approximate circumference of
radius of 3 inches?

a circle that has a radius of 4 feet?

A. 6 inches.

A. 25.12 feet.

d = 2 · r = 2 · 3 in. = 6 in.

C = 2 π · r

≈ 2 · 3.14 · 4 ft.

Q. What’s the approximate area of a circle

≈ 25.12 ft.

that has a radius of 10 millimeters?

A. 314 square millimeters.

A = π · r 2

≈ 3.14 · (10 mm)2

≈ 3.14 · 100 mm2

≈ 314 mm2

15. What’s the approximate area and circum-

16. Figure out the approximate area and cir-
ference of a circle that has a radius of 3
cumference of a circle that has a radius of
kilometers?

12 yards.

Solve It

Solve It

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212 Part III: A Giant Step Forward: Intermediate Topics

17. Write down the approximate area and cir-

18. Find the approximate area and circumfer-
cumference of a circle with a diameter of
ence of a circle that has a diameter of

52 centimeters.

86 inches.

Solve It

Solve It

Building Solid Measurement Skills

Solids take you into the real world, the third dimension. One of the simplest solids is the cube, a solid with six identical square faces. To find the volume of a cube, use the following formula, where s is the length of the side of any one face (check out Figure 12-8): V = s 3

A box (also called a rectangular solid) is a solid with six rectangular faces. To find the volume of a box, use the following formula, where l is the length, w is the width, and h is the height (see Figure 12-9):

V = l · w · h

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Chapter 12: Shaping Up with Geometry 213

Cube
side ( s)

Figure 12-8:

The volume
of a cube
using the
length of a
side (s).

height ( h)

Figure 12-9:

The volume
of a box
using the
length (l),
width ( w)

width (w),
and height

(h).

length ( l)

A prism is a solid with two identical bases and a constant cross section — that is, whenever you slice a prism parallel to the bases, the cross section is the same size and shape as the bases. A cylinder is a solid with two identical circular bases and a constant cross section. To find the volume of a prism or cylinder, use the following formula, where Ab is the area of the base and h is the height (see Figure 12-10). You can find the area formulas throughout this chapter:

V = Ab · h

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214 Part III: A Giant Step Forward: Intermediate Topics

Prism

Cylinder

Figure 12-10:

The volume
area of
of a prism or
base ( A )

b
cylinder
height ( h)

using the
height ( h)

area of
area of the
base ( A )

b
base (Ab)

and the
height (h).

A pyramid is a solid that has a base that’s a polygon (a shape with straight sides), with straight lines that extend from the sides of the base to meet at a single point. Similarly, a cone is a solid that has a base that’s a circle, with straight lines extending from every point on the edge of the base to meet at a single point. The formula for the volume of a pyramid is the same as for the volume of a cone. In this formula, illustrated in Figure 12-11, Ab is the area of the base, and h is the height:

V = 1⁄3 · Ab · h

Pyramid

Cone

Figure 12-11:

area of

The volume
base ( A )

b
of a pyramid
height ( h)

or cone
using the
height ( h)

area of
area of the
base ( A )

b
base (Ab)

and the
height (h).

Volume measurements are usually in cubic units, such as cubic centimeters (cm3) or cubic feet (ft.3).

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Chapter 12: Shaping Up with Geometry 215

Q. What’s the volume of a cube with a side

Q. Find the volume of a pyramid with a
that measures 4 centimeters?

square base whose side is 10 inches and
with a height of 6 inches.

A. 64 cubic centimeters.

A. 200 cubic inches. First, find the area of

V = s 3 = (4 cm)3 = 4 cm · 4 cm · 4 cm
the base using the formula for the area of

= 64 cm3
a square:

Q.

A

Calculate the volume of a box with a
b = s 2 = (10 in.)2 = 100 in.2
length of 7 inches, a width of 4 inches,

Now plug this result into the formula for
and a height of 2 inches.

the volume of a pyramid/cone:

A. 56 cubic inches.

V = 1⁄3 · Ab · h

V = l · w · h = 7 in. · 4 in. · 2 in. = 56 in.3

= 1⁄3 · 100 in.2 · 6 in.

= 200 in.3

Q. Find the volume of a prism with a base
that has an area of 6 square centimeters

Q. Find the approximate volume of a cone
and a height of 3 centimeters.

with a base that has a radius of 2 meters

A.

and with a height of 3 meters.

18 cubic centimeters.

V = A

A. 12.56 cubic meters. First, find the
b · h = 6 cm2 · 3 cm = 18 cm3
approximate area of the base using the

Q.

formula for the area of a circle:

Figure out the approximate volume of a
cylinder with a base that has a radius of

Ab = π · r 2

2 feet and a height of 8 feet.

≈ 3.14 · (2 m)2

A. 100.48 cubic feet. To begin, find the

= 3.14 · 4 m2
approximate area of the base using the

= 12.56 m2
formula for the area of a circle:

A

Now plug this result into the formula for
b = π · r 2
the volume of a pyramid/cone:

≈ 3.14 · (2 ft.)2

V = 1⁄

≈

3 · Ab · h

3.14 · 4 ft.2

= 1⁄

≈

3 · 12.56 m2 · 3 m

12.56 ft.2

= 12.56 m3

Now plug this result into the formula for
the volume of a prism/cylinder:

V = Ab · h

= 12.56 ft.2 · 8 ft

= 100.48 ft.3

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216 Part III: A Giant Step Forward: Intermediate Topics

19. Find the volume of a cube that has a side of

20. Figure out the volume of a box with a

19 meters.

length of 18 centimeters, a width of 14 cen-
timeters, and a height of 10 centimeters.

Solve It

Solve It

21. Figure out the approximate volume of

22. Find the approximate volume of a cone
a cylinder whose base has a radius of
whose base has a radius of 3 inches and

7 millimeters and whose height is
whose height is 8 inches.

16 millimeters.

Solve It

Solve It

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Chapter 12: Shaping Up with Geometry 217

Answers to Problems in Shaping

Up with Geometry

The following are the answers to the practice questions presented in this chapter.

a 14 square centimeters. Use the triangle area formula:

A = 1⁄2 · b · h = 1⁄2 · 7 cm · 4 cm = 14 cm2
b 85 square kilometers. Plug in the numbers for the base and height of the triangle: A = 1⁄2 · b · h = 1⁄2 · 10 km · 17 km = 85 km2
c 396 square inches. First, convert feet to inches. Twelve inches are in 1 foot: 2 ft. = 24 in.

Now use the area formula for a triangle:

A = 1⁄2 · b · h

= 1⁄2 · 24 in · 33 in.

= 396 in.2

Note: If you instead converted from inches to feet, the answer 2.75 square feet is also correct.

d 5 miles. Use the Pythagorean theorem to find the value of c as follows: a 2 + b 2 = c 2

32 + 42 = c 2

9 + 16 = c 2

25 = c 2

When you multiply c by itself, the result is 25. Therefore,
c = 5 mi.

e 13 millimeters. Use the Pythagorean theorem to find the value of c: a 2 + b 2 = c 2

52 + 122 = c 2

25 + 144 = c 2

169 = c 2

When you multiply c by itself, the result is 169. The hypotenuse is longer than the longest leg, so c has to be greater than 12. Use trial and error, starting with 13: 132 = 169

Therefore, the hypotenuse is 13 mm.

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218 Part III: A Giant Step Forward: Intermediate Topics
f 17 feet. Use the Pythagorean theorem to find the value of c:

a 2 + b 2 = c 2

82 + 152 = c 2

64 + 225 = c 2

289 = c 2

When you multiply c by itself, the result is 289. The hypotenuse is longer than the longest leg, so c has to be greater than 15. Use trial and error, starting with 16: 162 = 256

172 = 289

Therefore, the hypotenuse is 17 ft.

g Area is 81 square miles; perimeter is 36 miles. Use the formulas for the area and perimeter of a square:

A = s 2 = (9 mi.)2 = 81 mi.2

P = 4 · s = 4 · 9 mi = 36 mi.

h Area is 961 square centimeters; perimeter is 124 centimeters. Plug in 31 cm for s in the formulas for the area and perimeter of a square.

A = s 2 = (31 cm)2 = 961 cm2

P = 4 · s = 4 · 31 cm = 124 cm
i Area is 50 square inches; perimeter is 30 inches. Plug the length and width into the area and perimeter formulas for a rectangle:

A = l · w = 10 in. · 5 in. = 50 in.2

P = 2 · ( l + w) = 2 · (10 in. + 5 in.) = 30 in.

j Area is 437 square kilometers; perimeter is 84 kilometers. Use the rectangle area and perimeter formulas:

A = l · w = 23 km · 19 km = 437 km2

P = 2 · ( l + w) = 2 · (23 km + 19 km) = 84 km
k 54 square meters. Use the parallelogram/rhombus area formula:

A = b · h = 9 m · 6 m = 54 m2
l 221 square yards. Use the parallelogram/rhombus area formula:

A = b · h = 17 yd. · 13 yd. = 221 yd.2
m 35 square feet. Plug your numbers into the trapezoid area formula:

A = 1⁄2 · ( b 1 + b 2) · h

= 1⁄2 · (6 ft. + 8 ft.) · 5 ft.

= 1⁄2 · 14 ft. · 5 ft.

= 35 ft.2

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Chapter 12: Shaping Up with Geometry 219
n 525 square millimeters. Use the trapezoid area formula:

A = 1⁄2 · ( b 1 + b 2) · h

= 1⁄2 · (15 mm + 35 mm) · 21 mm

= 1⁄2 · 50 mm · 21

= 525 mm2
o Approximate area is 28.26 square kilometers; approximate circumference is 18.84 kilometers. Use the area formula for a circle to find the area:

A = π · r 2

≈ 3.14 · (3 km)2

= 3.14 · 9 km2

= 28.26 km2

Use the circumference formula to find the circumference:

C = 2 π · r

≈ 2 · 3.14 · 3 km

= 18.84 km
p Approximate area is 452.16 square yards; approximate circumference is 75.36 yards.

Use the area formula for a circle to find the area:

A = π · r 2

≈ 3.14 · (12 yd.)2

= 3.14 · 144 yd.2

= 452.16 yd.2

Use the circumference formula to find the circumference:

C = 2 π · r

≈ 2 · 3.14 · 12 yd.

= 75.36 yd.

q Approximate area is 2,122.64 square centimeters; approximate circumference is 163.28
centimeters. The diameter is 52 cm, so the radius is half of that, which is 26 cm. Use the area formula for a circle to find the area:

A = π · r 2

≈ 3.14 · (26 cm)2

= 3.14 · 676 cm2

= 2,122.64 cm2

Use the circumference formula to find the circumference:

C = 2 π · r

≈ 2 · 3.14 · 26 cm

= 163.28 cm

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220 Part III: A Giant Step Forward: Intermediate Topics
r Approximate area is 5,805.86 square inches; approximate circumference is 270.04 inches.

The diameter is 86 in., so the radius is half of that, which is 43 in. Use the area formula for a circle to find the area:

A = π · r 2

≈ 3.14 · (43 in.)2

= 3.14 · 1,849 in.2

= 5,805.86 in.2

Use the circumference formula to find the circumference:

C = 2 π · r

≈ 2 · 3.14 · 43 in.

= 270.04 in.

s 6,859 cubic meters. Substitute 19 m for s in the cube volume formula: V = s 3 = (19 m)3 = 6,859 m3
t 2,520 cubic centimeters. Use the box area formula:

V = l · w · h

= 18 cm · 14 cm · 10 cm = 2,520 cm3
u Approximately 2,461.76 cubic millimeters. First, use the area formula for a circle to find the area of the base:

Ab = π · r 2

≈ 3.14 · (7 mm)2

= 3.14 · 49 mm2

= 153.86 mm2

Plug this result into the formula for the volume of a prism/cylinder:

V = Ab · h

= 153.86 mm2 · 16 mm = 2,461.76 mm3
v Approximately 75.36 cubic inches. Use the area formula for a circle to find the area of the base: Ab = π · r 2

≈ 3.14 · (3 in.)2

= 3.14 · 9 in.2

= 28.26 in.2

Plug this result into the formula for the volume of a pyramid/cone:

V = 1⁄3 · Ab · h

= 1⁄3 · 28.26 in.2 · 8 in. = 75.36 in.3

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Chapter 13

Getting Graphic:

Cartesian Graphs

In This Chapter

ᮣ Understanding the basics of the Cartesian graph

ᮣ Plotting points and drawing lines on the Cartesian graphs

A graph is a visual tool for providing information about numbers. Graphs commonly appear in business reports, sales brochures, newspapers, and magazines — any place where conveying numerical information quickly and clearly is important.

In this chapter, I show you how to work with the graph that’s most commonly used in mathematics: the Cartesian graph.

Getting the Point of the Cartesian Graph

In math, the most commonly used graph is the Cartesian graph (also called the Cartesian coordinate system or the Cartesian plane). The Cartesian graph is basically two number lines that cross at 0. These number lines are called the x-axis (which runs horizontally) and the y-axis (which runs vertically). These two axes (plural of axis) cross at a point called the origin.

Every point on the Cartesian graph is represented by a pair of numbers in parentheses, called a set of Cartesian coordinates (or an ordered pair). The first number is called the x-coordinate, and the second is called the y-coordinate.

To plot (locate) a point on the Cartesian graph, start at the origin (0,0) and follow the coordinates:

ߜ The x-coordinate: The first number tells you how far to go to the right (if positive) or left (if negative) along the x-axis.

ߜ The y-coordinate: The second number tells you how far to go up (if positive) or down (if negative) along the y-axis.

For example, to plot the point P = (3, –4), start at the origin and count three places to the right; then travel down four places and plot your point there.

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222 Part III: A Giant Step Forward: Intermediate Topics

Q. Plot the following points on the Cartesian graph:

a. A = (2, 5)

b. B = (–3, 1)

c. C = (–2, –4)

d. D = (6, 0)

e. E = (–5, –5)

f. F = (0, –1)

A.

y

A = (2,5)

B = (–3,1)

D = (6,0)

x

F = (0,–1)

C = (–2,–4)

E = (–5,–5)

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Chapter 13: Getting Graphic: Cartesian Graphs 223

Q. Write down the Cartesian coordinates of points G through L.

y

H

G

I
x

L

J

K

A. G = (4, 3), H = (–1, 5), I = (–4, 0), J = (–3, –3), K = (0, –5), and L = (5, –1).

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224 Part III: A Giant Step Forward: Intermediate Topics

1. Plot each of the following points on the Cartesian graph.

y
x
a. M = (5, 6)

b. N = (–6, –2)

c. O = (0, 0)

d. P = (–1, 2)

e. Q = (3, 0)

f. R = (0, 2)

Solve It

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Chapter 13: Getting Graphic: Cartesian Graphs 225

2. Write down the Cartesian coordinates for each point S through X.

y

U

T

S
x

X

V

W

Solve It

Drawing the Line on the Cartesian Graph

After you understand how to plot points on a Cartesian graph (see the preceding section), you can use this skill to draw lines that represent equations on the graph. To see how this works, it’s helpful to understand the concept of a function.

A function is a mathematical machine — often in the form y = some expression that involves x — that turns one number into another number. The number you start with is called the input, and the new number that comes out is the output. On a graph, the input is usually x, and the output is usually y.

A useful tool for understanding functions is an input-output table In such a table, you plug var-ious x-values into your formula and do the necessary calculations to find the corresponding y- values.

You can use the Cartesian coordinates from an input-output table to plot points on the graph (as I show you in the preceding section). When these points all line up, draw a straight line through them to represent the function on the graph. Note: Technically, you need to plot only two points to figure out where the line should go. Still, finding more points is good practice, and it’s important when you’re graphing a function that isn’t a straight line.

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226 Part III: A Giant Step Forward: Intermediate Topics

Q. Make an input-output table for the function y = x + 2 for the input values 0, 1, 2, 3, and 4. Then write down Cartesian coordinates for four points on the graph that are on this function.

A. (0, 2), (1, 3), (2, 4), (3, 5), and (4, 6). Here’s an input-output table for the function y = x + 2: Input Value x
x + 2

Output Value y

0

0 + 2

2

1

1 + 2

3

2

2 + 2

4

3

3 + 2

5

4

4 + 2

6

As you can see, the first column has the five input values 0, 1, 2, 3, and 4. In the second column, I substitute each of these four values for x in the expression x + 2. In the third column, I evaluate this expression to get the four output values. To get Cartesian coordinates for the function, pair up numbers from the first and third columns: (0, 2), (1, 3), (2, 4), (3, 5), and (4, 6).

Q. Draw the function y = x + 2 as a line on the Cartesian graph by plotting points using the coordinates (0, 2), (1, 3), (2, 4), (3, 5), and (4, 6).

A.

y
y = x + 2
x

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Chapter 13: Getting Graphic: Cartesian Graphs 227

3. Graph y = x – 1.

4. Graph y = 2 x.

a. Make an input-output table for the func-
a. Make an input-output table for the func-
tion y = x – 1 for the input values 0, 1, 2,
tion y = 2 x for the input values 0, 1, 2, 3,

3, and 4.

and 4. (The notation 2 x means 2 · x. )

b. Use this table to write down Cartesian
b. Use this table to write down Cartesian
coordinates for five points on this
coordinates for five points on this
function.

function.

c. Plot these five points to draw the func-
c. Plot these five points to draw the func-
tion y = x – 1 as a line on the graph.

tion y = 2 x as a line on the graph.

Solve It

Solve It

5. Graph y = 3 x – 5.

6. Graph y = x⁄2 + 3.

a. Make an input-output table for the func-
a. Make an input-output table for the func-
tion y = 3 x – 5 for the input values 0, 1, 2,
tion y = x⁄2 + 3 for the input values –2, 0, 2,

3, and 4. (The notation 3 x means 3 · x. )

and 4.

b. Use this table to write down Cartesian
b. Use this table to write down Cartesian
coordinates for five points on this
coordinates for four points on this
function.

function.

c. Plot these five points to draw the func-
c. Plot these four points to draw the func-
tion y = 3 x – 5 as a line on the graph.

tion y = x⁄2 + 3 as a line on the graph.

Solve It

Solve It

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228 Part III: A Giant Step Forward: Intermediate Topics

Answers to Problems in Getting Graphic:

Cartesian Graphs

The following are the answers to the practice questions presented in this chapter: a See the following graph
y

M = (5, 6)

P = (–1, 2)

R = (0, 2)

Q= (3, 0)

0 = (0, 0)

x

N = (–6, –2)

b S = (4, 0), T = (1, 3), U = (–5, 4), V = (–3, –4), W = (0, 4), and X = (1, –1).

c Graph y = x – 1.

a.

Here’s the input-output table for the function y = x – 1.

Input Value x
x – 1

Output Value y

0

0 – 1

–1

1

1 – 1

0

2

2 – 1

1

3

3 – 1

2

4

4 – 1

3

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Chapter 13: Getting Graphic: Cartesian Graphs 229
b. (0, –1), (1, 0), (2, 1), (3, 2), and (4, 3).

c. See the following graph.

y
y = x – 1
x
d Graph y = 2 x.

a. Make the input-output table for y = 2 x.

Input Value x

2x

Output Value y

0

2 · 0

0

1

2 · 1

2

2

2 · 2

4

3

2 · 3

6

4

2 · 4

8
b. (0, 0), (1, 2), (2, 4), (3, 6), and (4, 8).

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230 Part III: A Giant Step Forward: Intermediate Topics
c. See the following graph.

y
y = 2 x
x
e Graph y = 3 x – 5.

a. Here’s the input-output table for y = 3 x – 5.

Input Value x

3x – 5

Output Value y

0

(3 · 0) – 5

–5

1

(3 · 1) – 5

–2

2

(3 · 2) – 5

1

3

(3 · 3) – 5

4

4

(3 · 4) – 5

7
b. (0, –5), (1, –2), (2, 1), (3, 4), and (4, 7).

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Chapter 13: Getting Graphic: Cartesian Graphs 231
c. See the following graph.

y
y = 3 x – 5
x
f Graph y = x⁄2 + 3.

a. Fill in the input-output table for y = x⁄2 + 3:

Input Value x
x⁄2 + 3

Output Value y

–2

–2⁄2 + 3

2

0

0⁄2 + 3

3

2

2⁄2 + 3

4

4

4⁄2 + 3

5
b. (–2, 2), (0, 3), (2, 4), and (4, 5).

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232 Part III: A Giant Step Forward: Intermediate Topics
c. See the following graph.

y
y = + 3
x

2
x

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Part IV

The X Factor:

Introducing Algebra

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In this part . . .

You’ve arrived! Here, you discover algebra and how to
unmask that elusive little fellow, Mr. X. You find that
in algebra, a letter such as x (called a variable) stands
for an unknown number. I show you how to work with
variables and constants to form algebraic expressions.

You also discover how to solve an algebraic equation to
figure out the value of x.

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Chapter 14

Expressing Yourself with

Algebraic Expressions

In This Chapter

ᮣ Evaluating algebraic expressions

ᮣ Breaking an expression into terms and identifying similar terms

ᮣ Applying the Big Four operations to algebraic terms

ᮣ Simplifying and FOILing expressions

In arithmetic, sometimes a box, a blank space, or a question mark stands for an unknown number, as in 2 + 3 = ❑ , 9 – ___ = 5, or ? · 6 = 18. But in algebra, a letter such as x stands for an unknown number. A letter in algebra is called a variable because its value can vary from one problem to the next. For instance, in the equation 10 – x = 8, x has a value of 2 because 10 – 2 = 8. But in the equation 2( x) = 12, x has a value of 6 because 2 · 6 = 12.

Here are a few algebra conventions you should know:

ߜ Multiplication: When multiplying by a variable, you rarely use the multiplication operator × or ·. As in arithmetic, you can use parentheses without an operator to express multiplication. For example, 3( x) means 3 · x. Often, the multiplication operator is dropped altogether. For example, 3 x means 3 · x.

ߜ Division: The fraction bar replaces the division sign, so to express x ÷ 5, you write x
either or
x⁄5.

5

ߜ Powers: Algebra commonly uses exponents to show a variable multiplied by itself. So to express x · x, you write x 2 rather than xx. To express xxx, you write x 3.

An exponent applies only to the variable that it follows, so in the expression 2 xy 2, only the y is squared. To make the exponent apply to both variables, you’d have to group them in parentheses, as in 2( xy)2.

In this chapter, you find out how to read, evaluate, break down, and simplify basic algebraic expressions. I also show you how to add, subtract, and multiply algebraic expressions.

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236 Part IV: The X Factor: Introducing Algebra

Plug It In: Evaluating Algebraic Expressions

An arithmetic expression is any sequence of numbers and operators that makes sense when placed on one side of an equal sign. An algebraic expression is similar, except that it includes at least one variable (such as x). Just as with arithmetic expressions, an algebraic expression can be evaluated — that is, reduced to a single number. In algebra, however, the answer depends on the value of the variable.

Here’s how to evaluate an algebraic expression when you’re given a value for every variable: 1. Substitute the proper value for every variable in the expression.

2. Evaluate the expression using the order of operations, as I show you in Chapter 4.

Q. Evaluate the algebraic expression

Q. Evaluate the algebraic expression
x 2 – 2 x + 5 when x = 3.

3 x 2 + 2 xy 4 – y 3 when x = 5 and y = 2.

A. 8. First, substitute 3 for x in the

A. 227. Plug in 5 for x and 3 for y in the
expression:

expression:

x 2 – 2 x + 5 = 32 – 2(3) + 5

3 x 2 + 2 xy 4 – y 3 = 3(52) + 2(5)(24) – 23

Evaluate the expression using the order

Evaluate the expression using the order
of operations. Start by evaluating the
of operations. Start by evaluating the
power:

three powers:

= 9 – 2(3) + 5

= 3(25) + 2(5)(16) – 8

Next, evaluate the multiplication:

Next, evaluate the multiplication:

= 9 – 6 + 5

= 75 + 160 – 8

Finally, evaluate the addition and sub-

Finally, evaluate the addition and sub-
traction from left to right:

traction from left to right:

= 3 + 5 = 8

= 235 – 8 = 227

1. Evaluate the expression x 2 + 5 x + 4 when

2. Find the value of 5 x 4 + x 3 – x 2 + 10 x + 8 when x = 3.

x = –2.

Solve It

Solve It

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Chapter 14: Expressing Yourself with Algebraic Expressions 237

3. Evaluate the expression x( x 2 – 6)( x – 7)

4
x

( −9)

when x = 4.

4. Evaluate when
x = 6.

3
x

( + 4)

Solve It

Solve It

5. Find the value of 3 x 2 + 5 xy + 4 y 2 when x = 5

6. Evaluate the expression x 6 y – 5 xy 2 when
and y = 7.

x = –1 and y = 9.

Solve It

Solve It

Knowing the Terms of Separation

Algebraic expressions begin to make more sense when you understand how they’re put together, and the best way to understand this is to take them apart and know what each part is called. Every algebraic expression can be broken into one or more terms. A term is any chunk of symbols set off from the rest of the expression by either an addition or subtraction sign. For example,

ߜ The expression –7 x + 2 xy has two terms: –7 x and 2 xy.

ߜ The expression x 4 – –
x 2

2 x + 2 has four terms: x 4, ,

– x 2 –2 x, and 2.

5

5

ߜ The expression 8 x 2 y 3 z 4 has only one term: 8 x 2 y 3 z 4.

When you separate an algebraic expression into terms, group the plus or minus sign with the term that immediately follows it. Then you can rearrange terms in any order you like. After the terms are separated, you can drop the plus signs.

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238 Part IV: The X Factor: Introducing Algebra

When a term doesn’t have a variable, it’s called a constant. ( Constant is just a fancy word for number when you’re talking about terms in an expression.) When a term has a variable, it’s called an algebraic term. Every algebraic term has two parts:

ߜ The coefficient is the signed numerical part of a term — that is, the number and the sign (+ or –) that go with that term. Typically, the coefficients 1 and –1 are dropped from a term, so when a term appears to have no coefficient, its coefficient is either 1 or –1, depending on the sign of that term. The coefficient of a constant is the constant itself.

ߜ The variable part is everything else other than the coefficient.

When two terms have the same variable part, they’re called similar terms, or like terms. For two terms to be similar, both the letters and their exponents have to be exact matches.

Check out some examples:

Variable Part

Examples of Similar Terms
x

2 x

–12 x

834 x
x 2

5 x 2

3⁄4 x 2

19.5 x 2
xy

–17 xy

1,000 xy

5⁄8 xy
x 3 yz 2

100 x 3 yz 2

–33.3 x 3 yz 2

–x 3 yz 2

None

7

–4.9

3⁄10

Notice that constant terms are all similar to each other because they all have no variable part.

Q. In the expression 3 x 2 – 2 x – 9, identify

Q. Identify the coefficient of every term in
which terms are algebraic and which are
the expression 2 x 4 – 5 x 3 + x 2 – x – 9.

constants.

A. 2, –5, 1, –1, and –9. The expression has

A. 3 x2 and –2 x are both algebraic terms,
five terms: 2 x 4, –5 x 3, x 2, – x, – 9. The coeffi-and –9 is a constant.

cient of 2 x 4 is 2, and the coefficient of

– 5 x 3 is –5. The term x 2 appears to have
no coefficient, so its coefficient is 1. The
term – x also appears to have no coeffi-
cient, so its coefficient it –1. The term –9
is a constant, so its coefficient is –9.

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Chapter 14: Expressing Yourself with Algebraic Expressions 239

7. Write down all the terms in the expression

8. Identify the coefficient of every term in

7 x 2 yz – 10 xy 2 z + 4 xyz 2 + y – z + 2. Which are

–2 x 5 + 6 x 4 + x 3 – x 2 – 8 x + 17.

algebraic terms, and which are constants?

Solve It

Solve It

9. Name every coefficient in the expression

10. In the expression 12 x 3 + 7 x 2 – 2 x – x 2 – 8 x 4 +

– x 3 y 3 z 3 – x 2 y 2 z 2 + xyz – x.

99 x + 99, identify any sets of similar terms.

Solve It

Solve It

Adding and Subtracting Similar Terms

You can add and subtract only similar algebraic terms (see the preceding section ). In other words, the variable parts need to match. To add two similar terms, simply add the coefficients and keep the variable parts of the terms the same. Subtraction works much the same: Find the difference between their coefficients and keep the same variable part.

Q. What is 3 x + 5 x?

Q. What is 24 x 3 – 7 x 3?

A. 8 x. The variable part of both terms is x,

A. 17 x3. Subtract the coefficients:

so you can add:

24 x 3 – 7 x 3 = (24 – 7) x 3 = 17 x 3

3 x + 5 x = (3 + 5) x = 8 x

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240 Part IV: The X Factor: Introducing Algebra

11. Add 4 x 2 y + –9 x 2 y.

12. Add x 3 y 2 + 20 x 3 y 2.

Solve It

Solve It

13. Subtract –2 xy 4 – 20 xy 4.

14. Subtract – xyz – (– xyz).

Solve It

Solve It

Multiplying and Dividing Terms

Unlike addition and subtraction, you can multiply any two algebraic terms, whether they’re similar or nonsimilar. Multiply two terms by multiplying their coefficients and collecting all the variables in each term into a single term. (When you collect the variables, you’re simply using exponents to give a count of how many x’ s, y’ s, and so on appeared in the original terms.)

A fast way to multiply variables with exponents is to add the exponents of identical variables together.

You can also divide any two algebraic terms. Division in algebra is usually represented as a fraction. Dividing is similar to reducing a fraction to lowest terms:

1. Reduce the coefficients to lowest terms as you would any other fraction (see Chapter 6).

2. Cancel out variables that appear in both the numerator and the denominator.

A fast way to divide is to subtract the exponents of identical variables. For each variable, subtract the exponent in the numerator minus the exponent in the denominator.

When a resulting exponent is a negative number, move that variable to the denominator and remove the minus sign.

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Chapter 14: Expressing Yourself with Algebraic Expressions 241

Q. What is 2 x(6 y)?

Q. What is ( x 2 y 3)( xy 5)( x 4)?

A. 12 xy. To get the final coefficient, multiply

A. x7 y8. Add exponents of the three x’ s (2 +
the coefficients 2 · 6 = 12. To get the vari-

1 + 4 = 7) to get the exponent of x in the
able part of the answer, combine the
answer (remember that x means x 1). Add
variables x and y:

the two y exponents (3 + 5 = 8) to get the
exponent of y in the answer.

2 x(6 y) = 2(6) xy = 12 xy

6 3 2
x y

Q.

Q. What is

?

What is 4 x(–9 x)?

3

2
xy

A. –36 x2. To get the final coefficient, multi-

A. 2 x2. Exponents mean repeated multipli-
ply the coefficients 4 · –9 = –36. To get
cation, so
the variable part of the answer, collect

3

2
the variables into a single term:

6 x y

6 xxxyy

=

3

2
xy

3 xyy

4 x(–9 x) = 4(–9) xx

Both the numerator and the denominator

Remember that x 2 is shorthand for xx, so
are divisible by 2, so reduce the coeffi-
rewrite the answer as follows:

cients just as you’d reduce a fraction:

–36 x 2

2 xxxyy

=
xyy

Q. What is 2 x(4 xy)(5 y 2)?

Now cancel out any variables repeated in

A. 40 x2 y3. Multiply all three coefficients both the numerator and denominator —
together and gather up the variables:

that is, one x and two y’s both above and
below the fraction bar:

2 x(4 xy)(5 y 2) = 2(4)(5) xxyy 2 = 40 x 2 y 3

2 xx x y y

In this answer, the exponent 2 that’s

=

= 2 xx
associated with x is just a count how
x y y
many x’ s appear in the problem; the
same is true of the exponent 3 that’s

Rewrite this result using an exponent.

associated with y.

2 x 2

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242 Part IV: The X Factor: Introducing Algebra

15. Multiply 4 x(7 x 2).

16. Multiply – xy 3 z 4(10 x 2 y 2 z)(–2 xz).

Solve It

Solve It

4

5

2

17.

6 x y

7 x y

Divide .

18. Divide .

8 4 4
x y

21

3
xy

Solve It

Solve It

Simplifying Expressions by

Combining Similar Terms

Although an algebraic expression may have any number of terms, you can sometimes make it smaller and easier to work with. This process is called simplifying an expression. To simplify, you combine similar terms, which means you add or subtract any terms that have identical variable parts. (To understand how to identify similar algebraic terms, see “Knowing the Terms of Separation,” earlier in this chapter. “Adding and Subtracting Similar Terms” shows you how to do the math.)

In some expressions, similar terms may not all be next to each other. In this case, you may want to rearrange the terms to place all similar terms together before combining them.

When rearranging terms, you must keep the sign (+ or –) with the term that follows it.

To finish, you usually rearrange the answer alphabetically, from highest exponent to lowest, and place any constant last. In other words, if your answer is –5 + 4 y 3 + x 2 + 2 x – 3 xy, you’d rearrange it to read x 2 + 2 x – 3 xy + 4 y 3 – 5. (This step doesn’t change the answer, but it kind of cleans it up, so teachers love it.)

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Chapter 14: Expressing Yourself with Algebraic Expressions 243

Q. Simplify the expression x 2 + 2 x – 7 x + 1.

Q. Simplify the expression 4 x 2 – 3 x + 2 + x –

7 x 2.

A. x2 – 5 x + 1. The terms 2 x and –7 x are similar, so you can combine them:

A. –3 x2 – 2 x + 2. I’ve begun by underlining the five terms of the expression:

2 x – 7 x = (2 – 7) x = –5 x

4 x 2 – 3 x + 2 + x – 7 x 2

Rearrange these terms as needed so that
all similar terms are together:

4 x 2 – 7 x 2 – 3 x + x + 2

At this point, combine the two under-
lined pairs of similar terms:

(4 – 7) x 2 + (–3 + 1) x + 2 = –3 x 2 – 2 x + 2

19. Simplify the expression 3 x 2 + 5 x 2 + 2 x – 8 x – 1.

20. Simplify the expression 6 x 3 – x 2 + 2 – 5 x 2 –

1 + x.

Solve It

Solve It

21. Simplify the expression 2 x 4 – 2 x 3 + 2 x 2 – x +

22. Simplify the expression x 5 – x 3 + xy – 5 x 3 –

9 + x + 7 x 2.

1 + x 3 – xy + x.

Solve It

Solve It

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244 Part IV: The X Factor: Introducing Algebra

Simplifying Expressions with Parentheses

When an expression contains parentheses, you need to get rid of the parentheses before you can simplify the expression. Here are the four possible cases:

ߜ Parentheses preceded by a plus sign (+): Just remove the parentheses. After that, you may be able to simplify the expression further by combining similar terms, as I show you in the preceding section.

ߜ Parentheses preceded by a minus sign (–): Change every term inside the parentheses to the opposite sign; then remove the parentheses. After the parentheses are gone, combine similar terms.

ߜ Parentheses preceded by no sign (a term directly next to a set of parentheses): Multiply the term next to the parentheses by every term inside the parentheses (make sure you include the plus or minus signs in your terms); then drop the parentheses.

Simplify by combining similar terms.

To multiply identical variables, simply add the exponents. For instance, x( x 2) = x 1 + 2 = x 3.

Likewise, – x 3( x 4) = –( x 3 + 4) = – x 7.

ߜ Two sets of adjacent parentheses: I discuss this case in the next section.

Q. Simplify the expression

Q. Simplify the expression

7 x + ( x 2 – 6 x + 4) – 5.

x – 3 x( x 3 – 4 x 2 + 2) + 8 x 4.

A. x2 + x – 1. Because a plus sign precedes A. 5 x4 + 12 x3 – 5 x. The term –3 x precedes this set of parentheses, you can drop the
this set of parentheses without a sign in
parentheses.

between, so multiply every term inside
the parentheses by –3 x and then drop the

7 x + ( x 2 – 6 x + 4) – 5
parentheses:

= 7 x + x 2 – 6 x + 4 – 5
x – 3 x( x 3 – 4 x 2 + 2) + 8 x 4

Now combine similar terms. I do this in

= x + –3 x( x 3) + –3 x(–4 x 2) + –3 x(2) + 8 x 4
two steps:

= x – 3 x 4 + 12 x 3 – 6 x + 8 x 4

= 7 x – 6 x + x 2 + 4 – 5

Now combine similar terms. I do this in

= x + x 2 – 1
two steps:

Finally, rearrange the answer from high-

= x – 6 x – 3 x 4 + 8 x 4 + 12 x 3
est exponent to lowest:

= –5 x + 5 x 4 + 12 x 3

= x 2 + x – 1

As a final step, arrange the exponents
from highest to lowest:

= 5 x 4 + 12 x 3 – 5 x

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Chapter 14: Expressing Yourself with Algebraic Expressions 245

23. Simplify the expression 3 x 3 + (12 x 3 – 6 x) +

24. Simplify the expression 2 x 4 – (–9 x 2 + x) +

(5 – x).

( x + 10).

Solve It

Solve It

25. Simplify the expression x – ( x 3 – x – 5) +

26. Simplify the expression – x 3( x 2 + x) – ( x 5 – x 4).

3( x 2 – x).

Solve It

Solve It

FOILing: Dealing with Two Sets of Parentheses

When an expression has two sets of parentheses next to each other, you need to multiply every term inside the first set of parentheses by every term in the second set. This process is called FOILing. The word FOIL is a memory device for the words First, Outside, Inside, Last, which helps keep track of the multiplication when both sets of parentheses have two terms each.

When two sets of adjacent parentheses are part of a larger expression, FOIL the contents of the parentheses and place the results into one set of parentheses. Then remove this set of parentheses using one of the rules I show you in the preceding section.

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246 Part IV: The X Factor: Introducing Algebra

Q. Simplify the expression ( x + 4)( x – 3).

Q. Simplify the expression x 2 – (–2 x + 5)

(3 x – 1) + 9.

A. x2 + x – 12. Start by multiplying the two first terms:

A. 7 x2 – 17 x + 14. Begin by FOILing the parentheses. Start by multiplying the

( x + 4)( x – 3)

x · x = x 2
two first terms:

Next, multiply the two outside terms:

(–2 x + 5)(3 x – 1)

–2 x · 3 x = –6 x 2

( x + 4)( x – 3)

x · –3 = –3 x

Multiply the two outside terms:

Now multiply the two inside terms:

(–2 x + 5)(3 x – 1)

–2 x · –1 = 2 x

( x + 4)( x – 3)

4 · x = 4 x

Multiply the two inside terms:

Finally, multiply the two last terms:

(–2 x + 5)(3 x – 1)

5 · 3 x = 15 x

( x + 4)( x – 3)

4 · –3 = –12

Finally, multiply the two last terms:

Add all four results together and simplify

(–2 x + 5)(3 x – 1)

5 · –1 = –5
by combining similar terms:

Add these four products together and put

( x + 4)( x – 3) = x 2 – 3 x + 4 x – 12 =
the result inside one set of parentheses,
x 2 + x – 12
replacing the two sets of parentheses that
were originally there:

x 2 – (–2 x + 5)(3 x – 1) + 9

= x 2 – (–6 x 2 + 2 x + 15 x – 5) + 9

The remaining set of parentheses is pre-
ceded by a minus sign, so change the
sign of every term in there to its opposite
and drop the parentheses:

= x 2 + 6 x 2 – 2 x – 15 x + 5 + 9

At this point, you can simplify the expres-
sion by combining similar terms. I do this
in two steps:

= x 2 + 6 x 2 – 2 x – 15 x + 5 + 9

= 7 x 2 – 17 x + 14

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Chapter 14: Expressing Yourself with Algebraic Expressions 247

27. Simplify the expression ( x + 7)( x – 2).

28. Simplify the expression ( x – 1)(– x – 9).

Solve It

Solve It

29. Simplify the expression

30. Simplify the expression

6 x – ( x – 2)( x – 4) + 7 x 2.

3 – 4 x( x 2 + 1)( x – 5) + 2 x 3.

Solve It

Solve It

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248 Part IV: The X Factor: Introducing Algebra

Answers to Problems in Expressing Yourself
with Algebraic Expressions

The following are the answers to the practice questions presented in this chapter.

a x 2 + 5 x + 4 = 28. Substitute 3 for x in the expression: x 2 + 5 x + 4 = 32 + 5(3) + 4

Evaluate the expression using the order of operations. Start with the power:

= 9 + 5(3) + 4

Continue by evaluating the multiplication:

= 9 + 15 + 4

Finish up by evaluating the addition from left to right:

= 24 + 4 = 28
b 5 x 4 + x 3 – x 2 + 10 x + 8 = 56. Plug in –2 for every x in the expression: 5 x 4 + x 3 – x 2 + 10 x + 8 = 5(–2)4 + (–2)3 – (–2)2 + 10(–2) + 8

Evaluate the expression using the order of operations. Start with the powers:

= 5(16) + –8 – 4 + 10(–2) + 8

Do the multiplication:

= 80 + –8 – 4 + –20 + 8

Finish up by evaluating the addition and subtraction from left to right:

= 72 – 4 + –20 + 8 = 68 + –20 + 8 = 48 + 8 = 56
c x( x 2 – 6)( x – 7) = –120. Substitute 4 for x in the expression: x( x 2 – 6)( x – 7) = 4 (42 – 6) (4 – 7)

Follow the order of operations as you evaluate the expression. Starting inside the first set of parentheses, evaluate the power and then the subtraction:

= 4(16 – 6)(4 – 7) = 4(10)(4 – 7)

Find the contents of the remaining set of parentheses:

= 4(10)(–3)

Evaluate the multiplication from left to right:

= 40(–3) = –120

4
x

( −9)

d

= 81⁄1,000 (or 0.081). Replace every x in the expression with a 6: 3
x

( + 4)

4

4
x

( −9)

(6−9)

3

=

3
x

( +4)

(6+4)

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Chapter 14: Expressing Yourself with Algebraic Expressions 249

Follow the order of operations. Evaluate the contents of the set of parentheses in the numerator and then in the denominator:

(− 4

4

3)

(−3)

=

3

=

(

3

6 + 4)

10

Continue by evaluating the powers from top to bottom:

= 81

81

=

103

1,000

You can also express this fraction as the decimal 0.081.

e 3 x 2 + 5 xy + 4 y 2 = 446. Substitute 5 for x and 7 for y in the expression: 3 x 2 + 5 xy + 4 y 2 = 3(5)2 + 5(5)(7) + 4(7)2

Evaluate using the order of operations. Start with the two powers:

= 3(25) + 5(5)(7) + 4(49)

Evaluate the multiplication from left to right:

= 75 + 175 + 196

Finally, do the addition from left to right:

= 250 + 196 = 446
f x 6 y – 5 xy 2 = 414. Plug in –1 for each x and 9 for each y in the expression: x 6 y – 5 xy 2 = (–1)6(9) – 5(–1)(9)2

Follow the order of operations. Start by evaluating the two powers:

= 1(9) – 5(–1)(81)

Continue by evaluating the multiplication from left to right:

= 9 – (–5)(81) = 9 – (–405)

Finally, do the subtraction:

= 414
g In 7 x 2 yz – 10 xy 2 z + 4 xyz 2 + y – z + 2, the algebraic terms are 7 x2 yz, –10 xy2 z, 4 xyz2, y, and – z; the constant is 2.

h In –2 x 5 + 6 x 4 + x 3 – x 2 – 8 x + 17, the six coefficients, in order, are –2, 6, 1, –1, –8, and 17.

i In – x 3 y 3 z 3 – x 2 y 2 z 2 + xyz – x, the four coefficients, in order, are –1, –1, 1, and –1.

j In 12 x 3 + 7 x 2 – 2 x – x 2 – 8 x 4 + 99 x + 99, 7 x2 and – x2 are similar to each other (the variable part is x 2); – 2 x and 99 x are also similar to each other (the variable part is x).

k 4 x 2 y + –9 x 2 y = (4 + –9) x 2 y = –5 x2 y.

l x 3 y 2 + 20 x 3 y 2 = (1 + 20) x 3 y 2 = 21 x3 y2.

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250 Part IV: The X Factor: Introducing Algebra
m –2 xy 4 – 20 xy 4 = (–2 – 20) xy 4 = –22 xy4.

n – xyz – (– xyz) = [–1 – (–1)] xyz = (–1 + 1) xyz = 0.

o 4 x(7 x 2) = 28 x3. Multiply the two coefficients to get the coefficient of the answer; then gather the variables into one term:

4 x(7 x 2) = 4(7) xx 2 = 28 x 3
p – xy 3 z 4(10 x 2 y 2 z)(–2 xz) = 20 x4 y5 z6. Multiply the coefficients (–1 · 10 · –2 = 20) to get the coefficient of the answer. Add the x exponents (1 + 2 + 1 = 4) to get the exponent of x in the answer. Add the y exponents (3 + 2 = 5) to get the exponent of y in the answer. And add the z exponents (4 + 1 + 1 = 6) to get the exponent of z in the answer.

6 4 5
x y

3 y
q

=

4

4

. Reduce the coefficients of the numerator and denominator just as you’d 8 x y

4
reduce a fraction:

6 4 5
x y

3 4 5
x y

=

8 4 4
x y

4 4 4
x y

To get the x exponent of the answer, take the x exponent in the numerator minus the x exponent in the denominator: 4 – 4 = 0, so the x’ s cancel:

3 5
y

= 4 4
y

To get the y exponent of the answer, take the y exponent in the numerator minus the y exponent in the denominator: 5 – 4 = 1, so you have only y 1, or y, in the numerator: 3 y

= 4

7 2
x y
r

=
x

3

3 . Reduce the coefficients of the numerator and denominator just as you’d 21 xy

3 y
reduce a fraction:

7 2

2
x y
x y

=

21

3
xy

3

3
xy

Take the x exponent in the numerator minus the x exponent in the denominator (2 – 1 = 1) to get the x exponent of the answer:

xy

= 3 y 3

To get the y exponent, take the y exponent in the numerator minus the y exponent in the denominator (1 – 3 = –2):

xy −2

=

3

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Chapter 14: Expressing Yourself with Algebraic Expressions 251

To finish up, remove the minus sign from the y exponent and move this variable to the denomonator:

x

= 3 y 2
s 3 x 2 + 5 x 2 + 2 x – 8 x – 1= 8 x2 – 6 x – 1. Combine the following underlined similar terms: 3 x 2 + 5 x 2 + 2 x – 8 x – 1

= 8 x 2 – 6 x – 1
t 6 x 3 – x 2 + 2 – 5 x 2 – 1 + x = 6 x3 – 6 x2 + x + 1. Rearrange the terms so similar terms are next to each other:

6 x 3 – x 2 + 2 – 5 x 2 – 1 + x

= 6 x 3 – x 2 – 5 x 2 + x + 2 – 1

Now combine the underlined similar terms:

= 6 x 3 – 6 x 2 + x + 1
u 2 x 4 – 2 x 3 + 2 x 2 – x + 9 + x + 7 x 2 = 2 x4 – 2 x3 + 9 x2 + 9. Put similar terms next to each other: 2 x 4 – 2 x 3 + 2 x 2 – x + 9 + x + 7 x 2

= 2 x 4 – 2 x 3 + 2 x 2 + 7 x 2 – x + x + 9

Now combine the underlined similar terms:

= 2 x 4 – 2 x 3 + 9 x 2 + 9

Note that the two x terms cancel each other out.

v x 5 – x 3 + xy – 5 x 3 – 1 + x 3 – xy + x = x5 – 5 x3 + x – 1. Rearrange the terms so similar terms are next to each other:

x 5 – x 3 + xy – 5 x 3 – 1 + x 3 – xy + x

= x 5 – x 3 – 5 x 3 + x 3 + xy – xy + x – 1

Now combine the underlined similar terms:

= x 5 – 5 x 3 + x – 1

Note that the two xy terms cancel each other out.

w 3 x 3 + (12 x 3 – 6 x) + (5 – x) = 15 x3 – 7 x + 5. A plus sign precedes both sets of parentheses, so you can drop both sets:

3 x 3 + (12 x 3 – 6 x) + (5 – x)

= 3 x 3 + 12 x 3 – 6 x + 5 – x

Now combine similar terms:

= 3 x 3 + 12 x 3 – 6 x – x + 5

= 15 x 3 – 7 x + 5

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252 Part IV: The X Factor: Introducing Algebra
x 2 x 4 – (–9 x 2 + x) + ( x + 10) = 2 x4 + 9 x2 + 10. A minus sign precedes the first set of parentheses, so change the sign of every term inside this set and then drop these parentheses: 2 x 4 – (–9 x 2 + x) + ( x + 10)

= 2 x 4 + 9 x 2 – x + ( x + 10)

A plus sign precedes the second set of parentheses, so just drop this set:

= 2 x 4 + 9 x 2 – x + x + 10

Now combine similar terms:

= 2 x 4 + 9 x 2 + 10
y x – ( x 3 – x – 5) + 3( x 2 – x) = – x3 + 3 x2 – x + 5. A minus sign precedes the first set of parentheses, so change the sign of every term inside this set and then drop these parentheses: x – ( x 3 – x – 5) + 3( x 2 – x)

= x – x 3 + x + 5 + 3( x 2 – x)

You have no sign between the term 3 and the second set of parentheses, so multiply every term inside these parentheses by 3 and then drop the parentheses:

= x – x 3 + x + 5 + 3 x 2 – 3 x

Now combine similar terms:

= x + x – 3 x – x 3 + 5 + 3 x 2

= – x – x 3 + 5 + 3 x 2

Rearrange the answer so that the exponents are in descending order:

= – x 3 + 3 x 2 – x + 5

A – x 3( x 2 + x) – ( x 5 – x 4) = –2 x5. You have no sign between the term – x 3 and the first set of parentheses, so multiply – x 3 by every term inside this set and then drop these parentheses:

– x 3( x 2 + x) – ( x 5 – x 4)

= – x 5 – x 4 – ( x 5 – x 4)

A minus sign precedes the second set of parentheses, so change every term inside this set and then drop the parentheses:

= – x 5 – x 4 – x 5 + x 4

Combine similar terms, noting that the x 4 terms cancel out:

= – x 5 – x 5 – x 4 + x 4

= –2 x 5

B ( x + 7)( x – 2) = x2 + 5 x – 14. Begin by FOILing the parentheses. Start by multiplying the two first terms:

( x + 7)( x – 2)

x · x = x 2

Multiply the two outside terms:

( x + 7)( x – 2)

x · –2 = –2 x

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Chapter 14: Expressing Yourself with Algebraic Expressions 253

Multiply the two inside terms:

( x + 7)( x – 2)

7 · x = 7 x

Finally, multiply the two last terms:

( x + 7)( x – 2)

7 · –2 = –14

Add these four products together and simplify by combining similar terms:

x 2 – 2 x + 7 x – 14 = x 2 + 5 x – 14

C ( x – 1)(– x – 9) = – x2 – 8 x + 9. FOIL the parentheses. Multiply the two first terms, the two outside terms, the two inside terms, and the two last terms:

( x – 1)(– x – 9)

x · – x = – x 2

( x – 1)(– x – 9)

x · –9 = –9 x

( x – 1)(– x – 9)

–1 · – x = x

( x – 1)(– x – 9)

–1 · –9 = 9

Add these four products together and simplify by combining similar terms:

– x 2 – 9 x + x + 9 = – x 2 – 8 x + 9

D 6 x – ( x – 2)( x – 4) + 7 x 2 = 6 x2 + 12 x – 8. Begin by FOILing the parentheses: Multiply the first, outside, inside, and last terms:

( x – 2)( x – 4)

x · x = x 2

( x – 2)( x – 4)

x · –4 = –4 x

( x – 2)( x – 4)

–2 · x = –2 x

( x – 2)( x – 4)

–2 · –4 = 8

Add these four products together and put the result inside one set of parentheses, replacing the two sets of parentheses that were originally there:

6 x – ( x – 2)( x – 4) + 7 x 2 = 6 x – ( x 2 – 4 x – 2 x + 8) + 7 x 2

The remaining set of parentheses is preceded by a minus sign, so change the sign of every term in there to its opposite and drop the parentheses:

= 6 x – x 2 + 4 x + 2 x – 8 + 7 x 2

Now simplify the expression by combining similar terms and reordering your solution:

= 6 x + 4 x + 2 x – x 2 + 7 x 2 – 8

= 6 x 2 + 12 x – 8

E 3 – 4 x( x 2 + 1)( x – 5) + 2 x 3 = –4 x4 + 22 x3 – 4 x2 + 20 x + 3. Begin by FOILing the parentheses, multiplying the first, outside, inside, and last terms:

( x 2 + 1)( x – 5)

x 2 · x = x 3

( x 2 + 1)( x – 5)

x 2 · –5 = –5 x 2

( x 2 + 1)( x – 5)

1 · x = x

( x 2 + 1)( x – 5)

1 · –5 = –5

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254 Part IV: The X Factor: Introducing Algebra

Add these four products together and put the result inside one set of parentheses, replacing the two sets of parentheses that were originally there:

3 – 4 x( x 2 + 1)( x – 5) + 2 x 3 = 3 – 4 x( x 3 – 5 x 2 + x – 5) + 2 x 3

The remaining set of parentheses is preceded by the term –4 x with no sign in between, so multiply –4 x by every term in there and then drop the parentheses:

= 3 – 4 x 4 + 20 x 3 – 4 x 2 + 20 x + 2 x 3

Now simplify the expression by combining similar terms:

= –4 x 4 + 20 x 3 + 2 x 3 – 4 x 2 + 20 x + 3

= –4 x 4 + 22 x 3 – 4 x 2 + 20 x + 3

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Chapter 15

Finding the Right Balance:

Solving Algebraic Equations

In This Chapter

ᮣ Solving simple algebraic equations without algebra

ᮣ Understanding the balance scale method

ᮣ Moving terms across the equal sign

ᮣ Using cross-multiplication to simplify equations

In this chapter, you use your skills in doing the Big Four operations and simplifying algebraic expressions (see Chapter 14) to solve algebraic equations — that is, equations with one or more variables (such as x). Solving an equation means you figure out the value of the variable. First, I show you how to solve very simple equations for x without using algebra.

Then, as the problems get tougher, I show you a variety of methods to figure out the value of x.

Solving Simple Algebraic Equations

You don’t always need algebra to solve an algebraic equation. Here are three ways to solve simpler problems:

ߜ Inspection: For the very simplest algebra problems, inspection — just looking at the problem — is enough. The answer just jumps out at you.

ߜ Rewriting the problem: In slightly harder problems, you may be able to rewrite the problem so you can find the answer. In some cases, this involves using inverse operations; in other cases, you can use the same operation with the numbers switched around. (I introduce inverse operations in Chapter 2.)

ߜ Guessing and checking: When problems are just a wee bit tougher, you can try guessing the answer and then checking to see whether you’re right. Check by substituting your guess for x.

When your guess is wrong, you can usually tell whether it’s too high or too low. Use this information to guide your next guess.

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256 Part IV: The X Factor: Introducing Algebra

In some cases, you can simplify the problem before you begin solving. On either side of the equal sign, you can rearrange the terms and combine similar terms as I show you in Chapter 14. After the equation is simplified, use any method you like to find x.

As you simplify, don’t move any terms to the opposite side of the equal sign. Also, don’t combine similar terms on opposite sides of the equal sign.

Q. In the equation x + 3 = 10, what’s the

Because 140 is greater than 108, this
value of x?

guess was too high. But now you know
that the right answer is between 10 and

A. x = 7. You can solve this problem

20. So now try x = 15:

through simple inspection. Because

8(15) – 20 = 120 – 20 = 100

7 + 3 = 10, x = 7.

The number 100 is only a little less than

Q. Solve the equation 7 x = 224 for x.

108, so this guess was only a little too
low. Try x = 16:

A. x = 32. Turn the problem around using

8(16) – 20 = 128 – 20 = 108
the inverse of multiplication, which is
division:

The answer 108 is correct, so x = 16.

7 · x = 224 means 224 ÷ 7 = x

Q. Solve for x: 8 x 2 – x + x 2 + 4 x – 9 x 2 = 18.

Now you can solve the problem easily
using long division (I don’t show this

A. x = 6. Rearrange the expression on the
step, but for practice with long division,
left side of the equation so that all simi-
see Chapter 1):

lar terms are next to each other:

224 ÷ 7 = 32

8 x 2 + x 2 – 9 x 2 – x + 4 x = 18

Q. Find the value of x in the equation

Combine similar terms:

8 x – 20 = 108.

3 x = 18

A. x = 16. Guess what you think the answer

Notice that the x 2 terms cancel each
may be. For example, perhaps it’s x = 10:

other out. Because 3(6) = 18, you know

8(10) – 20 = 80 – 20 = 60
that x = 6.

Because 60 is less than 108, that guess
was too low. Try a higher number — say,
x = 20:

8(20) – 20 = 160 – 20 = 140

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Chapter 15: Finding the Right Balance: Solving Algebraic Equations 257

1. Solve for x in each case just by looking at

2. Use the correct inverse operation to
the equation.

rewrite and solve each problem.

a. x + 5 = 13
a. x + 41 = 97
b. 18 – x = 12
b. 100 – x = 58
c. 4 x = 44
c. 13 x = 273
d. 30⁄ x = 3
d. 238⁄ x = 17

Solve It

Solve It

3. Find the value of x in each equation by

4. Simplify the equation and then solve for x
guessing and checking.

using any method you like:

a. 19 x + 22 = 136
a. x 5 – 16 + x + 20 – x 5 = 24
b. 12 x – 17 = 151
b. 5 xy + x – 2 xy + 27 – 3 xy = 73
c. 19 x – 8 = 600
c. 6 x – 3 + x 2 – x + 8 – 5 x = 30
d. x 2 + 3 = 292
d. –3 + x 3 + 4 + x – x 3 – 1 = 2 xy + 7 – x – 2 xy + x Solve It

Solve It

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258 Part IV: The X Factor: Introducing Algebra

Equality for All: Using the Balance

Scale to Isolate x

Think of an equation as a balance scale, one of the classic ones that has a horizontal beam with little weighing pans hanging from each end. The equal sign means that both sides hold the same amount and therefore balance each other out. To keep that equal sign, you have to maintain that balance. Therefore, whatever you do to one side of the equation, you have to do to the other.

For example, the equation 4 + 2 = 6 is in balance because both sides are equal. If you want to add 1 to one side of the equation, you need to add 1 to the other side to keep the balance.

Notice that the equation stays balanced, because each side equals 7:

4 + 2 + 1 = 6 + 1

You can apply any of the Big Four operations to the equation, provided that you keep the equation balanced at all times. For example, here’s how you multiply both sides of the original equation by 10. Note that the equation stays balanced, because each side equals 60: 10(4 + 2) = 10(6)

The simple concept of the balance scale is the heart and soul of algebra. Algebra is really just a bunch of tricks to make sure that the scale never goes out of balance. When you understand how to keep the scale in balance, you can solve algebraic equations by isolating x —
that is, by getting x alone on one side of the equation and everything else on the other side.

For most basic equations, isolating x is a three-step process:

1. Add or subtract the same number from each side to get all constants (non- x terms) on one side of the equation.

On the other side of the equation, the constants should cancel each other out and equal 0.

2. Add or subtract to get all x terms on the other side of the equation.

The x term that’s still on the same side as the constant should cancel out.

3. Divide to isolate x.

Q. Use the balance scale method to find the

The right side of the equation still con-
value of x in the equation 5 x – 6 = 3 x + 8.

tains a 3 x. To get all x terms on the left
side of the equation, subtract 3 x from

A. x = 7. To get all constants on the right
both sides:

side of the equation, add 6 to both sides,

5 x = 3 x +14
which causes the –6 to cancel out on the

3

− x −3 x
left side of the equation:

2 x =

14

5 x − 6 = 3 x + 8

+6

+ 6

Divide by 2 to isolate x:

5 x

= 3 x +14

2 x

14

=

2

2
x

= 7

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Chapter 15: Finding the Right Balance: Solving Algebraic Equations 259

5. Use the balance scale method to find the

6. Solve the equation 10 x – 10 = 8 x + 12 using
value of x in the equation 9 x – 2 = 6 x + 7.

the balance scale method.

Solve It

Solve It

7. Find the value of x in 4 x – 17 = x + 22.

8. Solve for x: 15 x – 40 = 11 x + 4.

Solve It

Solve It

Switching Sides: Rearranging

Equations to Isolate x

When you understand how to keep equations in balance (as I show you in the preceding section), you can use a quicker method to solve algebra problems. The shortcut is to rearrange the equation by placing all x terms on one side of the equal sign and all constants (non- x terms) on the other side. Essentially, you’re doing the addition and subtraction without showing it. You can then isolate x.

Like the balance scale method, solving for x by rearranging the equation is a three-step process; however, the steps usually take less time to write:

1. Rearrange the terms of the equation so that all x terms are on one side of the equation and all constants (non- x terms) are on the other side.

When you move a term from one side of the equal sign to the other, always negate that term. That is, if the term is positive, make it negative; if the term is negative, make it positive.

2. Combine similar terms on both sides of the equation.

3. Divide to isolate x.

When one or both sides of the equation contain parentheses, remove them (as I show you in Chapter 14). Then use these three steps to solve for x.

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260 Part IV: The X Factor: Introducing Algebra

Q. Find the value of x in the equation

Q. Find the value of x in the equation

7 x – 6 = 4 x + 9.

3 – (7 x – 13) = 5(3 – x) – x.

A. x = 5. Rearrange the terms of the equa-

A. x = 1. Before you can begin rearranging
tion so that the x terms are on one side
terms, remove the parentheses on both
and the constants are on the other. I do
sides of the equation. On the left side,
this in two steps:

the parentheses are preceded by a minus
sign, so change the sign of every term

7 x – 6 = 4 x + 9
and remove the parentheses:

7 x = 4 x + 9 + 6

3 – 7 x + 13 = 5(3 – x) – x

7 x – 4 x = 9 + 6

On the right side, no sign comes between

Combine similar terms on both sides of
the 5 and the parentheses, so multiply
the equation:

every term inside the parentheses by 5
and remove the parentheses:

3 x = 15

3 – 7 x + 13 = 15 – 5 x – x

Divide by 3 to isolate x:

Now you can solve the equation in three

3 x

15

=
steps. Put the x terms on one side and

3

3
the constants on the other, remembering
x

= 5
to switch the signs as needed:

–7 x = 15 – 5 x – x – 3 – 13

–7 x + 5 x + x = 15 – 3 – 13

Combine similar terms on both sides of
the equation:

– x = –1

Divide by –1 to isolate x:

− x = −1

−1

−1
x = 1

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Chapter 15: Finding the Right Balance: Solving Algebraic Equations 261

9. Rearrange the equation 10 x + 5 = 3 x + 19 to

10. Find the value of x by rearranging the equa-
solve for x.

tion 4 + (2 x + 6) = 7( x – 5).

Solve It

Solve It

11. Solve –[2( x + 7) + 1] = x – 12 for x.

12. Find the value of x: – x 3 + 2( x 2 + 2 x + 1) = 4 x 2

– ( x 3 + 2 x 2 – 18).

Solve It

Solve It

Barring Fractions: Cross-Multiplying
to Simplify Equations

Fraction bars, like parentheses, are grouping symbols: The numerator is one group, and the denominator is another. But like parentheses, fraction bars can block you from rearranging an equation and combining similar terms. Luckily, cross-multiplication is a great trick for removing fraction bars from an algebraic equation.

You can use cross-multiplication to compare fractions, as I show you in Chapter 6. To show that fractions are equal, you can cross-multiply them — that is, multiply the numerator of one fraction by the denominator of the other. For example, here are two equal fractions. As you can see, when you cross-multiply them, the result is another balanced equation: 2⁄5 = 4⁄10

2(10) = 4(5)

20 = 20

You can use this trick to simplify algebraic equations that contain fractions.

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262 Part IV: The X Factor: Introducing Algebra

Q. Use cross-multiplication to solve the

Q. Use cross-multiplication to solve the
equation 2 x⁄ 3 = x – 3.

2 x + 1

6 x
equation .

=
x + 1

3 x + 1

A. x = 9. Cross-multiply to get rid of the

A. x = 1. In some cases, after you cross-
fraction in this equation. To do this, turn
multiply, you may need to FOIL one or
the right side of the equation into a frac-
both sides of the resulting equation.

tion by inserting a denominator of 1:

First, cross-multiply to get rid of the

2 x
x

3
fraction bar in this equation:

=

−

3

1

(2 x + 1)(3 x + 1) = 6 x( x + 1)

Now cross-multiply:

Now remove the parentheses on the left

2 x(1) = 3( x – 3)

side of the equation by FOILing (as I
show you in Chapter 14):

Remove the parentheses from both sides

6 x 2 + 2 x + 3 x + 1 = 6 x( x + 1)

(as I show you in Chapter 14):

2 x = 3 x – 9

To remove the parentheses from the
right side, multiply 6 x by every term

At this point, you can rearrange the
inside the parentheses, and then drop
equation and solve for x, as I show you
the parentheses:

earlier in this chapter:

6 x 2 + 2 x + 3 x + 1 = 6 x 2 + 6 x

2 x − 3 x =

9

−

At this point, you can rearrange the

− x =

9

−
equation and solve for x:

− x

9

= −

1 = 6 x 2 + 6 x – 6 x 2 – 2 x – 3 x

1

−

1

−
x = 9

Notice that the two x 2 terms cancel each
other out:

1 = 6 x – 2 x – 3 x

1 = x

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Chapter 15: Finding the Right Balance: Solving Algebraic Equations 263

13.

+

14.

Rearrange the equation x

5
x

= − to

Find the value of x by rearranging the
solve for x.

2

8

+
equation .

3 x

5

= x −1

7

Solve It

Solve It

15.

x

2 x

3

16. Find the value of x in this equation:

Solve the equation

=

+ .

2 x − 5

4 x −7

2 x + 3

6
x .

=

−

Solve It

4 − 8 x

4 x + 8

Solve It

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264 Part IV: The X Factor: Introducing Algebra

Answers to Problems in Finding the Right

Balance: Solving Algebraic Equations

The following are the answers to the practice questions presented in this chapter.

a Solve for x in each case just by looking at the equation.

a. x + 5 = 13; x = 8, because 8 + 5 = 13.

b. 18 – x = 12; x = 6, because 18 – 6 = 12
c. 4 x = 44; x = 11, because 4(11) = 44
d. 30⁄ x = 3; x = 10, because 30⁄10 = 3
b Use the correct inverse operation to rewrite and solve each problem.

a. x + 41 = 97; x = 56. Change the addition to subtraction: x + 41 = 97 is the same as 97 – 41 = x, so x = 56.

b. 100 – x = 58; x = 42. Change the subtraction to addition: 100 – x = 58 means the same thing as 100 – 58 = x, so x = 42.

c. 13 x = 273; x = 21. Change the multiplication to division: 13 x = 273 is equivalent to 273⁄13 = x, so x = 21.

d. 238⁄ x = 17; x = 14. Switch around the division: 238⁄ x = 17 means 238⁄17 = x, so x = 14.

c Find the value of x in each equation by guessing and checking.

a. 19 x + 22 = 136; x = 6. First, try x = 10: 19(10) + 22 = 190 + 22 = 212

212 is greater than 136, so this guess is too high. Try x = 5:

19(5) + 22 = 95 + 22 = 117

117 is only a little less than 136, so this guess is a little too low. Try x = 6: 19 (6) + 22 = 114 + 22 = 136

136 is correct, so x = 6.

b. 12 x – 17 = 151; x = 14. First, try x = 10: 12(10) – 17 = 120 – 17 = 103

103 is less than 151, so this guess is too low. Try x = 20:

12(20) – 17 = 240 – 17 = 223

223 is greater than 151, so this guess is too high. Therefore, x is between 10 and 20. Try x = 15: 12(15) – 17 = 180 – 17 = 163

163 is a little greater than 151, so this guess is a little too high. Try x = 14: 12(14) – 17 = 168 – 17 = 151

151 is correct, so x = 14.

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Chapter 15: Finding the Right Balance: Solving Algebraic Equations 265
c. 19 x – 8 = 600; x = 32. First, try x = 10: 19(10) – 8 = 190 – 8 = 182

182 is much less than 600, so this guess is much too low. Try x = 30: 19(30) – 8 = 570 – 8 = 562

562 is still less than 600, so this guess is still too low. Try x = 35: 19(35) – 8 = 665 – 8 = 657

657 is greater than 600, so this guess is too high. Therefore, x is between 30 and 35. Try x = 32: 19(32) – 8 = 608 – 8 = 600

600 is correct, so x = 32.

d. x 2 + 3 = 292; x = 17. First, try x = 10: 102 +3 = 100 + 3 = 103

103 is less than 292, so this guess is too low. Try x = 20:

202 + 3 = 400 + 3 = 403

403 is greater than 292, so this guess is too high. Therefore, x is between 10 and 20. Try x = 15: 152 + 3 = 225 + 3 = 228

228 is less than 292, so this guess is too low. Therefore, x is between 15 and 20. Try x = 17: 172 + 3 = 289 + 3 = 292

292 is correct, so x = 17.

d Simplify the equation and then solve for x using any method you like: a. x 5 – 16 + x + 20 – x 5 = 24; x = 20. Rearrange the expression on the left side of the equation so that all similar terms are next to each other:

x 5 – x 5 + x + 20 – 16 = 24

Combine similar terms:

x + 4 = 24

Notice that the two x 5 terms cancel each other out. Because 20 + 4 = 24, you know that x = 20.

b. 5 xy + x – 2 xy + 27 – 3 xy = 73; x = 46. Rearrange the expression on the left side of the equation: 5 xy – 2 xy – 3 xy + x + 27 = 73

Combine similar terms:

x + 27 = 73

Notice that the three xy terms cancel each other out. Because x + 27 = 73 means 73 – 27 = x, you know that x = 46.

c. 6 x – 3 + x 2 – x + 8 – 5 x = 30; x = 5. Rearrange the expression on the left side of the equation so that all similar terms are adjacent:

6 x – x – 5 x + x 2 + 8 – 3 = 30

Combine similar terms:

x 2 + 5 = 30

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266 Part IV: The X Factor: Introducing Algebra

Notice that the three x terms cancel each other out. Try x = 10: 102 + 5 = 100 + 5 = 105

105 is greater than 30, so this guess is too high. Therefore, x is between 0 and 10. Try x = 5: 52 + 5 = 25 + 5 = 30

30 is correct, so x = 5.

d. –3 + x 3 + 4 + x – x 3 – 1 = 2 xy + 7 – x – 2 xy + x; x = 7. Rearrange the expression on the left side of the equation:

–3 + 4 – 1 + x 3 – x 3 + x = 2 xy + 7 – x – 2 xy + x Combine similar terms:

x = 2 xy + 7 – x – 2 xy + x

Notice that the three constant terms cancel each other out, and so do the two x 3 terms. Now rearrange the expression on the right side of the equation:

x = 2 xy – 2 xy + 7 – x + x

Combine similar terms:

x = 7

Notice that the two xy terms cancel each other out, and so do the two x terms. Therefore, x = 7.

e x = 3. To get all constants on the right side of the equation, add 2 to both sides: 9 x − 2 = 6 x + 7

+ 2

+ 2

9 x

= 6 x + 9

To get all x terms on the left side, subtract 6 x from both sides: 9 x

=

6 x + 9

− 6 x

6

− x

3 x

=

9

Divide by 3 to isolate x:

3 x

9

=

3

3
x

= 3
f x = 11. Move all constants on the right side of the equation by adding 10 to both sides: 10 x −10 = 8 x + 12

+10

+10

10 x

= 8 x + 22

To get all x terms on the left side, subtract 8 x from both sides: 10 x

=

8 x + 22

8

− x

8

− x

2 x

=

22

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Chapter 15: Finding the Right Balance: Solving Algebraic Equations 267

Divide by 2 to isolate x:

2 x

22

=

2

2
x

= 11
g x = 13. Add 17 to both sides to get all constants on the right side of the equation: 4 x −17 =
x + 22

+17

+17

4 x

= x + 39

Subtract x from both sides to get all x terms on the left side: 4 x

=
x + 39

− x

− x

3 x

=

39

Divide by 3 to isolate x:

3 x

39

=

3

3
x

= 13
h x = 11. To get all constants on the right side of the equation, add 40 to both sides: 15 x − 40 =

11 x + 4

+ 40

+ 40

15 x

= 11 x + 44

To get all x terms on the left side, subtract 11 x from both sides: 15 x = 11 x + 44

11

− x −11 x

4 x =

44

Divide by 4 to isolate x:

4 x

44

=

4

4
x

= 11
i x = 2. Rearrange the terms of the equation so that the x terms are on one side and the constants are on the other. I do this in two steps:

10 x + 5 = 3 x + 19

10 x = 3 x + 19 – 5

10 x – 3 x = 19 – 5

Combine similar terms on both sides:

7 x = 14

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268 Part IV: The X Factor: Introducing Algebra

Divide by 7 to isolate x:

7 x

14

=

7

7
x

= 2
j x = 9. Before you can begin rearranging terms, remove the parentheses on both sides of the equation. On the left side, the parentheses are preceded by a plus sign, so just drop them: 4 + (2 x+ 6) = 7( x – 5)

4 + 2 x + 6 = 7( x – 5)

On the right side, no sign comes between the number 7 and the parentheses, so multiply 7 by every term inside the parentheses and then drop the parentheses:

4 + 2 x + 6 = 7 x – 35

Now you can solve for x by rearranging the terms of the equation. Group the x terms on one side and the constants on the other. I do this in two steps:

4 + 6 = 7 x – 35 – 2 x

4 + 6 + 35 = 7 x– 2 x

Combine similar terms on both sides:

45 = 5 x

Divide by 5 to isolate x:

45

5

=
x

5

5

9

=
x
k x = –1. Before you can begin rearranging terms, remove the parentheses on the left side of the equation. Start with the inner parentheses, multiplying 2 by every term inside that set:

–[2( x + 7) + 1] = x – 12

–[2 x + 14 + 1] = x – 12

Next, remove the remaining parentheses, switching the sign of every term within that set:

–2 x – 14 – 1 = x – 12

Now you can solve for x by rearranging the terms of the equation:

–2 x – 14 – 1 + 12 = x

–14 – 1 + 12 = x + 2 x

Combine similar terms on both sides:

–3 = 3 x

Divide by 3 to isolate x:

−3 = 3 x

3

3

−1 =
x

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Chapter 15: Finding the Right Balance: Solving Algebraic Equations 269
l x = 4. Before you can begin rearranging terms, multiply the terms in the left-hand parentheses by 2 and remove the parentheses on both sides of the equation:

– x 3 + 2( x 2 + 2 x + 1) = 4 x 2 – ( x 3 + 2 x 2 – 18)

– x 3 + 2 x 2 + 4 x + 2 = 4 x 2 – ( x 3 + 2 x 2 – 18)

– x 3 + 2 x 2 + 4 x + 2 = 4 x 2 – x 3 – 2 x 2 + 18

Rearrange the terms of the equation:

– x 3 + 2 x 2 + 4 x + 2 – 4 x 2 + x 3 + 2 x 2 = 18

– x 3 + 2 x 2 + 4 x – 4 x 2 + x 3 + 2 x 2 = 18 – 2

Combine similar terms on both sides (notice that the x 3 and x 2 terms all cancel out): 4 x = 16

Divide by 4 to isolate x:

4 x

16

=

4

4
x

= 4
m x = –4. Remove the fraction from the equation by cross-multiplying: x + 5
x

= −

2

8

8( x + 5) = –2 x

Multiply to remove the parentheses from the left side of the equation:

8 x + 40 = –2 x

At this point, you can solve for x:

40 =

2

− x − 8 x

40 =

10

− x

40

10

= − x

−10

10

−

− 4 = x
n x = 3. Change the right side of the equation to a fraction by attaching a denominator of 1.

Remove the fraction bar from the equation by cross-multiplying:

3 x + 5
x

1

=

−

7

1

3 x + 5 = 7( x – 1)

Multiply 7 by each term inside the parentheses to remove the parentheses from the right side of the equation:

3 x + 5 = 7 x – 7

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270 Part IV: The X Factor: Introducing Algebra

Now solve for x:

5 = 7 x – 7 – 3 x

5 + 7 = 7 x – 3 x

12 = 4 x

3 = x
o x = 5. Remove the fractions from the equation by cross-multiplying: x

2 x

3

=

+

2 x − 5

4 x −7
x(4 x – 7) = (2 x + 3)(2 x – 5)

Remove the parentheses from the left side of the equation by multiplying through by x; remove parentheses from the right side of the equation by FOILing:

4 x 2 – 7 x = 4 x 2 – 10 x + 6 x – 15

Rearrange the equation:

4 x 2 – 7 x – 4 x 2 + 10 x – 6 x = –15

Notice that the two x 2 terms cancel each other out:

−7 x +10 x − 6 x = −15

− 3 x = −15

−3 x = −15

−3

−3
x = 5
p x = 0. Remove the fractions from the equation by cross-multiplying: 2 x + 3

6
x

=

−

4 − 8 x

4 x + 8

(2 x + 3)(4 x + 8) = (6 – x)(4 + 8 x)

FOIL both sides of the equation to remove the parentheses:

8 x 2 + 16 x + 12 x + 24 = 24 – 48 x – 4 x + 8 x 2

At this point, rearrange terms so you can solve for x:

8 x 2 + 16 x + 12 x + 24 + 48 x + 4 x – 8 x 2 = 24

8 x 2 + 16 x + 12 x + 48 x + 4 x – 8 x 2 = 24 – 24

Notice that the x 2 terms and the constant terms drop out of the equation: 16 x +12 x + 48 x + 4 x = 0

80 x = 0

80 x

0

=

80

80
x = 0

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Part V

The Part of Tens

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In this part . . .

This part rewards you with a couple of top-ten lists that
give you some new perspectives on math. I show you
ten interesting ways to represent numbers, from Roman
numerals to binary numbers. I also show you some curi-
ous types of numbers that have a variety of interesting
properties.

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Chapter 16

Ten Alternative Numeral
and Number Systems

In This Chapter

ᮣ Looking at numeral systems of the Egyptians, Babylonians, Romans, and Mayans

ᮣ Comparing the decimal number system with the binary and hexadecimal systems

ᮣ Taking a leap into the world of prime-based numbers

The distinction between numbers and numerals is subtle but important. A number is an idea that expresses how much or how many. A numeral is a written symbol that expresses a number.

In this chapter, I show you ten ways to represent numbers that differ from the Hindu-Arabic (decimal) system. Some of these systems use entirely different symbols from those you’re used to; others use the symbols that you know in different ways. A few of these systems have useful applications, and the others are just curiosities. (If you like, you can always use them for sending secret messages!) In any case, you may find it fun and interesting to see how many different ways people have found to represent the numbers that you’re accustomed to.

Tally Marks

Numbers are abstractions that stand for real things. The first known numbers came into being with the rise of trading and commerce — people needed to keep track of commodities such as animals, harvested crops, or tools. At first, traders used clay or stone tokens to help simplify the job of counting. The first numbers were probably an attempt to simplify this recording system. Over time, tally marks scratched either in bone or on clay took the place of tokens.

When you think about it, the use of tally marks over tokens indicates an increase in sophisti-cation. Previously, one real object (a token) had represented another real object (for example, a sheep or ear of corn). After that, an abstraction (a scratch) represented a real object.

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274 Part V: The Part of Tens

Bundled Tally Marks

As early humans grew more comfortable letting tally marks stand for real-world objects, the next development in numbers was probably tally marks scratched in bundles of 5 (fingers on one hand), 10 (fingers on both hands), or 20 (fingers and toes). Bundling provided a simple way to count larger numbers more easily.

Of course, this system is much easier to read than non-bundled scratches — you can easily multiply or count by fives to get the total. Even today, people keep track of points in games using bundles such as these.

Egyptian Numerals

Ancient Egyptian numerals are among the oldest number systems still in use today. Egyptian numerals use seven symbols, explained in Table 16-1.

Table 16-1

Egyptian Numerals

Number

Symbol

1

Stroke

10

Yoke

100

Coil of Rope

1,000

Lotus

10,000

Finger

100,000

Frog

1,000,000

Man with Raised Hands

Numbers are formed by accumulating enough of the symbols that you need. For example, 7 = 7 strokes

24 = 2 yokes, 4 strokes

1,536 = 1 lotus, 5 coils of rope, 3 yokes, 6 strokes

In Egyptian numbers, the symbol for 1,000,000 also stands for infinity (ϱ).

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Chapter 16: Ten Alternative Numeral and Number Systems 275

Babylonian Numerals

Babylonian numerals, which came into being about 4,000 years ago, use two symbols: 1 = Y 10 = <

For numbers less than 60, numbers are formed by accumulating enough of the symbols you need. For example,

6 = YYYYYY

34 = <<<YYYY

59 = <<<<<YYYYYYYYY

For numbers 60 and beyond, Babylonian numerals use place value based on the number 60.

For example,

61 = Y Y

(one 60 and one 1)

124 = YY YYYY

(two 60s and four 1s)

611 = < <Y

(ten 60s and eleven 1s)

Unlike the decimal system that you’re used to, Babylonian numbers had no symbol for zero to serve as a placeholder, which causes some ambiguity. For example, the symbol for 60 is the same as the symbol for 1.

Ancient Greek Numerals

Ancient Greek numerals were based on the Greek letters. The numbers from 1 to 999 were formed using the symbols in Table 16-2.

Table 16-2

Numerals Based on the Greek Alphabet

Ones

Tens

Hundreds

1 = α (alpha)

10 = ι (iota)

100 = ρ (rho)

2 = β (beta)

20 = κ (kappa)

200 = σ (sigma)

3 = γ (gamma)

30 = λ (lambda)

300 = τ (tau)

4 = δ (delta)

40 = µ (mu)

400 = υ (upsilon)

5 = ε (epsilon)

50 = ν (nu)

500 = φ (phi)

6 =

(digamma)

60 = ξ (xi)

600 = χ (chi)

7 = ζ (zeta)

70 = ο (omicron)

700 = ψ (psi)

8 = η (eta)

80 = π (pi)

800 = ω (omega)

9 = θ (theta)

90 = ¥ (koppa)

900 = µ (sampi)

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276 Part V: The Part of Tens

Roman Numerals

Although Roman numerals are over 2,000 years old, people still use them today, either decoratively (for example, on clocks, cornerstones and Super Bowl memorabilia) or when numerals distinct from decimal numbers are needed (for example, in outlines).

Roman numerals use seven symbols, all of which are capital letters in the Latin alphabet (which pretty much happens to be the English alphabet as well):

I = 1

V = 5

X = 10

L = 50

C = 100

D = 500

M = 1,000

Most numbers are formed by accumulating enough of the symbol that you need. Generally, you list the symbols in order, from highest to lowest. Here are a few examples: 3 = III

8 = VIII

20 = XX

70 = LXX

300 = CCC

600 = DC

2,000 = MM

Numbers that would contain 4s or 9s in the decimal system are formed by transposing two numbers to indicate subtraction. When you see a smaller symbol come before a larger one, you have to subtract the smaller value from the number that comes after it:

4 = IV

9 = IX

40 = XL

90 = XC

400 = CD

900 = CM

These two methods of forming numbers are sufficient to represent all decimal numbers up to 3,999:

37 = XXXVII

664 = DCLXIV

1,776 = MDCCLXXVI

1,999 = MCMXCIX

Higher numbers are less frequent, but you form them by putting a bar over a symbol, beginning with a bar over V for 5,000 and ending with a bar over M for 1,000,000. The bar means that you need to multiply by 1,000.

Mayan Numerals

Mayan numerals developed in South America during roughly the same period that Roman numerals developed in Europe. Mayan numerals use two symbols: dots and horizontal bars.

A bar is equal to 5, and a dot is equal to 1. Numbers from 1 to 19 are formed by accumulating dots and bars. For example,

3 = 3 dots

7 = 2 dots over 1 bar

19 = 4 dots over 3 bars

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Chapter 16: Ten Alternative Numeral and Number Systems 277

Numbers from 20 to 399 are formed using these same combinations, but raised up to indicate place value. For example,

21 = raised 1 dot, 1 dot (one 20 + one 1)

86 = raised 4 dots, 1 dot over 1 bar (four 20s + six 1s)

399 = raised 4 dots over 3 bars, 4 dots over 3 bars (nineteen 20s + nineteen 1s) As you can see, Mayan place value is based on the number 20 rather than the number 10 that we use. Numbers from 400 to 7,999 are formed similarly, with an additional place — the 400s place.

Because Mayan numerals use place value, there’s no limit to the magnitude of numbers that you can express. This fact makes Mayan numerals more mathematically advanced than either Egyptian or Roman numerals. For example, you could potentially use Mayan numerals to represent astronomically large numbers — such as the number of stars in the known uni-verse or the number of atoms in your body — without changing the basic rules of the system. This sort of representation would be impossible with Egyptian or Roman numerals.

Base 2, or Binary Numbers

Binary numbers use only two symbols — 0 and 1. This simplicity makes binary numbers useful as the number system that computers use for data storage and computation.

Like the decimal system you’re most familiar with, binary numbers use place value (see Chapter 1 for more on place value). Unlike the decimal system, binary place value is based not on powers of ten (1, 10, 100, 1,000, and so forth) but on powers of two (20, 21, 22, 23, 24, 25, 26, 27, 28, 29, and so on), as seen in Table 16-3 (see Chapter 2 for more on powers.) Table 16-3

Binary Place Values

512s

256s

128s

64s

32s

16s

8s

4s

2s

1s

Notice that each number in the table is exactly twice the value of the number to its immediate right. Note also that the base-2 number system is based on the base of a bunch of exponents (see Chapter 2, which covers powers). You can use this chart to find out the decimal value of a binary number. For example, suppose you want to represent the binary number 1101101 as a decimal number. First, place the number in the binary chart, as in Table 16-4.

Table 16-4

Breaking Down a Binary Number

512s

256s

128s

64s

32s

16s

8s

4s

2s

1s

1

1

0

1

1

0

1

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278 Part V: The Part of Tens

The table tells you that this number consists of one 64, one 32, no 16s, one 8, one 4, no 2s, and one 1. Add these numbers up, and you find that the binary number 1101101 equals the decimal number 109:

64 + 32 + 8 + 4+ 1 = 109

To translate a decimal number into its binary equivalent, use whole-number division to get a quotient and a remainder (as I explain in Chapter 1). Start by dividing the number you’re translating into the next-highest power of 2. Keep dividing powers of 2 into the remainder of the result. For example, here’s how to find out how to represent the decimal number 83 as a binary number:

83 ÷ 64 = 1 r 19

19 ÷ 32 = 0 r 19

19 ÷ 16 = 1 r 3

3 ÷ 8 = 0 r 3

3 ÷ 4 = 0 r 3

3 ÷ 2 = 1 r 1

1 ÷ 1 = 1 r 0

So the decimal number 83 equals the binary number 1010011 because 64 + 16 + 2 + 1 = 83.

Base 16, or Hexadecimal Numbers

The computer’s first language is binary numbers. But in practice, humans find binary numbers of any significant length virtually undecipherable. Hexadecimal numbers, however, are readable to humans and still easily translated into binary numbers, so computer pro-grammers use hexadecimal numbers as a sort of common language when interfacing with computers at the deepest level, the level of hardware and software design.

The hexadecimal number system uses all ten digits 0 through 9 from the decimal system.

Additionally, it uses six more symbols:

A = 10

B = 11

C = 12

D = 13

E = 14

F = 15

Hexadecimal is a place-value system based on powers of 16, as shown in Table 16-5.

Table 16-5

Hexadecimal Place Values

1,048,576s

65,536s

4,096s

256s

16s

1s

As you can see, each number in the table is exactly 16 times the number to its immediate right.

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Chapter 16: Ten Alternative Numeral and Number Systems 279

Prime-Based Numbers

One wacky way to represent numbers unlike any of the others in this chapter is prime-based numbers. Prime-based numbers are similar to decimal, binary, and hexadecimal numbers (which I describe earlier) in that they use place value to determine the value of digits. But unlike these other number systems, prime-based numbers are based not on addition but on multiplication. Table 16-6 shows a place value chart for prime-based numbers.

Table 16-6

Prime-Based Place Values

31s

29s

23s

19s

17s

13s

11s

7s

5s

3s

2s

You can use the table to find the decimal value of a prime-based number. For example, suppose you want to represent the prime-based number 1204 as a decimal number. First, place the number in the table, as shown in Table 16-7.

Table 16-7

Breaking Down a Prime-Based Number

31s

29s

23s

19s

17s

13s

11s

7s

5s

3s

2s

1

2

0

4

As you may have guessed, the table tells you that this number consists of one 7, two 5s, no 3s, and four 2s. But instead of adding these numbers together, you multiply them: 7 · 5 · 5 · 2 · 2 · 2 · 2 = 2,800

To translate a decimal number into its prime-based equivalent, factor the number and place its factors into the chart. For example, suppose you want to represent the decimal number 60 as a prime-based number. First, decompose 60 into its prime factors (as I show you in Chapter 5):

60 = 2 · 2 · 3 · 5

Now count the number of twos, threes, and fives and place these in Table 16-6. The result should look like Table 16-8.

Table 16-8

Finding the Prime-Based Equivalent of 60

31s

29s

23s

19s

17s

13s

11s

7s

5s

3s

2s

1

1

2

So 60 in prime-based notation is 112.

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280 Part V: The Part of Tens

Interestingly, multiplication with prime-based numbers looks like addition with decimal numbers. For example, in decimal numbers, 9 · 10 = 90. The prime-based equivalents of the factors and product — 9, 10, and 90 — are 20, 101, and 121. So here’s how to do the same multiplication in prime-based numbers:

20 · 101 = 121

As you can see, this multiplication looks more like addition. Even weirder is that 1 in decimal notation is represented as 0 in prime-based notation. This makes sense when you think about it, because multiplying by 1 is very similar to adding 0.

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Chapter 17

Ten Curious Types of Numbers

In This Chapter

ᮣ Shaping up with square, triangular, and cubic numbers

ᮣ Perfecting your understanding of perfect numbers

ᮣ Getting on friendly terms with amicable numbers

ᮣ Becoming one with prime numbers

Numbers seem to have personalities all their own. For example, even numbers are go-along numbers that break in half so you can carry them more conveniently. Odd numbers are more stubborn and don’t break apart so easily. Powers of ten are big friendly numbers that are easy to add and multiply, whereas most other numbers are prickly and require special attention. In this chapter, I introduce you to some interesting types of numbers, with properties that other numbers don’t share.

Square Numbers

When you multiply any number by itself, the result is a square number. For example, 12 = 1 × 1 = 1

22 = 2 × 2 = 4

32 = 3 × 3 = 9

42 = 4 × 4 = 16

52 = 5 × 5 = 25

Therefore, the sequence of square numbers begins as follows:

1, 4, 9, 16, 25, . . .

To see why they’re called square numbers, look at the arrangement of coins in squares in Figure 17-1.

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282 Part V: The Part of Tens

Figure 17-1:

The first five
square
numbers.

12 = 1

22 = 4

32 = 9

42 = 16

52 = 25

What’s really cool about the list of square numbers is that you can get it by adding the odd numbers (3, 5, 7, 9, 11, 13, ...), beginning with 3, to each preceding number in the list:

Square Number

Preceding Number + Odd Number

Sum

22 = 4

12 + 3

1 + 3 = 4

32 = 9

22 + 5

4 + 5 = 9

42 = 16

32 + 7

9 + 7 = 16

52 = 25

42 + 9

16 + 9 = 25

62 = 36

52 + 11

25 + 11 = 36

72 = 49

62 + 13

36 + 13 = 49

Triangular Numbers

When you add up any sequence of consecutive positive numbers starting with 1, the result is a triangular number. For example,

1 = 1

1 + 2 = 3

1 + 2 + 3 = 6

1 + 2 + 3 + 4 = 10

1 + 2 + 3 + 4 + 5 = 15

So the sequence of triangular numbers begins as follows:

1, 3, 6, 10, 15, . . .

Triangular numbers’ shapely name makes sense when you begin arranging coins in triangles.

Check out Figure 17-2.

Figure 17-2:

The first five
triangular
numbers.

1

3

6

10

15

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Chapter 17: Ten Curious Types of Numbers 283

Cubic Numbers

If you’re feeling that the square and triangular numbers are too flat, add a dimension and begin playing with the cubic numbers. You can generate a cubic number by multiplying any number by itself three times:

13 = 1 × 1 × 1 = 1

23 = 2 × 2 × 2 = 8

33 = 3 × 3 × 3 = 27

43 = 4 × 4 × 4 = 64

53 = 5 × 5 × 5 = 125

The sequence of cubic numbers begins as follows:

1, 8, 27, 64, 125, . . .

Cubic numbers live up to their name. Look at the cubes in Figure 17-3.

Figure 17-3:

The first
five cubic
numbers.

13 = 1

23 = 8

33 = 27

43 = 64

53 = 125

Factorial Numbers

In math, the exclamation point (!) means factorial, so you read 1! as one factorial. You get a factorial number when you multiply any sequence of consecutive positive numbers, starting with the number itself and counting down to 1. For example,

1! = 1

2! = 2 × 1 = 2

3! = 3 × 2 × 1 = 6

4! = 4 × 3 × 2 × 1 = 24

5! = 5 × 4 × 3 × 2 × 1 = 120

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284 Part V: The Part of Tens

Thus, the sequence of factorial numbers begins as follows:

1, 2, 6, 24, 120, . . .

Factorial numbers are very useful in probability, which is the mathematics of how likely an event is to occur. With probability problems, you can figure out how likely you are to win the lottery or estimate your chances of guessing your friend’s locker combination within the first few tries.

Powers of Two

Multiplying the number 2 by itself repeatedly gives you the powers of two. For example, 21 = 2

22 = 2 × 2 = 4

23 = 2 × 2 × 2 = 8

24 = 2 × 2 × 2 × 2 = 16

25 = 2 × 2 × 2 × 2 × 2 = 32

Powers of two are the basis of binary numbers (see Chapter 16), which are important in computer applications. They’re also useful for understanding Fermat numbers, which I discuss later in this chapter.

Perfect Numbers

Any number that equals the sum of its own factors (excluding itself) is a perfect number. To see how this works, find all the factors of 6 (as I show you in Chapter 5):

6: 1, 2, 3, 6

Now add up all these factors except 6:

1 + 2 + 3 = 6

These factors add up to the number you started with, so 6 is a perfect number. The next perfect number is 28. First, find all the factors of 28:

28: 1, 2, 4, 7, 14, 28

Now add up all these factors except 28:

1 + 2 + 4 + 7 + 14 = 28

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Chapter 17: Ten Curious Types of Numbers 285

Again, these factors add up to the number you started with, so 28 is a perfect number.

Perfect numbers are few and far between. The sequence of perfect numbers begins with the following five numbers:

6; 28; 496; 8,128; 33,550,336; . . .

You can use the same method I outline to check 496 and beyond by yourself.

Amicable Numbers

Amicable numbers are similar to perfect numbers, except they come in pairs. The sum of the factors of one number (excluding the number itself) is equal to the second number, and vice versa. For example, one amicable pair is 220 and 284. To see why, first find all the factors of each number:

220: 1, 2, 4, 5, 10, 11, 22, 44, 55, 110, 220

284: 1, 2, 4, 7, 42, 71, 142, 284

For each number, add up all the factors except the number itself:

1 + 2 + 4 + 5 + 10 + 11 + 22 + 44 + 55 + 110 = 284

1 + 2 + 4 + 7 + 42 + 71 + 142 = 220

Notice that the factors of 220 add up to 284, and the factors of 284 add up to 220. That’s what makes this pair of numbers amicable.

The next-lowest pair of amicable numbers is 1,184 and 1,210. You can either trust me on this one or do the calculation yourself.

Prime Numbers

Any number that has exactly two factors — 1 and itself — is called a prime number. For example, here are the first few prime numbers:

2, 3, 5, 7, 11, 13, 17, 19, . . .

The prime numbers are infinite in number — that is, they go on forever. See Chapter 5 for more on prime numbers.

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286 Part V: The Part of Tens

Mersenne Primes

Any number that’s 1 less than a power of two (which I discuss earlier in this chapter) is called a Mersenne number (named for French mathematician Marin Mersenne). Therefore, every Mersenne number is of the following form:

2 n – 1 (where n is a non-negative integer)

When a Mersenne number is also a prime number (see the preceding section), it’s called a Mersenne prime. For example,

22 – 1 = 4 – 1 = 3

23 – 1 = 8 – 1 = 7

25 – 1 = 32 – 1 = 31

27 – 1 = 128 – 1 = 127

213 – 1 = 8,192 – 1 = 8,191

Mersenne primes are of interest to mathematicians because they possess properties that ordinary prime numbers don’t have. One of these properties is that they tend to be easier to find than other prime numbers. For this reason, the search for the largest known prime number is usually a search for a Mersenne prime.

Fermat Primes

A Fermat number (named for mathematician Pierre de Fermat) is of the following form: 22^ n + 1 (where n is a non-negative integer)

The ^ symbol means that you’re finding a power, so with this formula, you first find 2 n; then you use that answer as an exponent on 2. For example, here are the first five Fermat numbers: 22^0 + 1 = 21 + 1 = 3

22^1 + 1 = 22 + 1 = 5

22^2 + 1 = 24 + 1 = 16 + 1 = 17

22^3 + 1 = 28 + 1 = 256 + 1 = 257

22^4 + 1 = 216 + 1 = 4,294,967,296 + 1 = 4,294,967,297

As you can see, Fermat numbers grow very quickly. When a Fermat number is also a prime number (see earlier in this chapter), it’s called a Fermat prime. As it happens, the five Fermat numbers are also Fermat primes (testing this is fairly simple for the first four numbers above and much harder for the fifth).

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Index

• Symbols and

1
as fraction denominator, 92

Numerics •
fractions equal to, 92
as neither prime nor composite, 73

≈ (approximately equals symbol), 30
rules for powers containing, 32

{ } (braces), 57. See also ( ) (parentheses)

2

[ ] (brackets), 57. See also ( ) (parentheses)

as base of binary numbers, 277–278

. (decimal point)

divisibility test for, 70
for money amounts less than one dollar, 141

Fermat numbers based on powers of, 286
moving to multiply or divide by powers of

Mersenne numbers based on powers of, 286
ten, 142
powers of, 284, 286

≠ (doesn’t equal symbol), 30

10, powers of. See powers of ten

. (dot) for multiplication, 2, 7, 235

16, as base of hexadecimal number

! (exclamation point) as factorial symbol, 283
system, 278

> (greater than symbol), 30

< (less than symbol), 30

• A •

– (minus sign)

adjacent minus signs, 39, 42
absolute value, 39, 40
for negative numbers, 37
adding. See also arithmetic expressions
outside absolute value bars, 39
associative property, 27

( ) (parentheses)

in balance scale method for solving equa-
in algebraic expressions, FOILing, 245–246
tions, 258
in algebraic expressions, removing, 244
column lineup for, 14, 146
answers to problems, 35, 61–67
commutative property, 24
in arithmetic expressions with powers,
decimal numbers, 146

56–57
exponents to multiply identical
in arithmetic expressions without powers,
variables, 244

54–55
fractions, 111–114
associative property use with, 27–29
as inverse of subtracting, 23–24
braces and brackets as forms of, 57
mixed numbers, 120–123
for math group operations, 27
with negative numbers, 40–41
nested, 57–58
number line for, 12, 13
order of operations for, 59
order of operations for, 59
when subtracting negative numbers, 42
similar terms, 239

× (times sign), dot used instead of, 2, 7, 235
algebra. See also algebraic expressions

0
answers to problems, 248–254, 264–270
absolute value of, 39
balance scale method for solving
leading, 8, 178
equations, 258
as placeholder, 8, 177
conventions in, 2, 7, 235
rules for powers containing, 32
cross-multiplying fractions in equations,
trailing, 142, 148, 149–150, 154

261–262

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288 Basic Math & Pre-Algebra Workbook For Dummies
algebra (continued)

triangle, 202, 203
guessing and checking method for solving
units of measurement for, 202
equations, 254, 255
arithmetic expressions
inspection method for solving equations,
with addition and/or subtraction only,

255, 256

49–50
rearranging equations to isolate x, 259–260
algebraic expressions versus, 236
rewriting equations, 254, 255
answers to problems, 60–67
simplifying expressions before solving
defined, 49
equations, 256
evaluate, defined, 49
solving an equation, defined, 255
mixed-operator, 52–53
variables, defined, 235
with multiplication and/or division only,
algebraic expressions

51–52
adding similar terms, 239
with nested parentheses, 57–58
algebraic terms in, 238
order of operations for, 49, 59
answers to problems, 248–254
with parentheses and powers, 56–57
arithmetic expressions versus, 236
with parentheses but no powers, 54–55
combining similar terms, 242–243
with powers but no parentheses, 53–54
constants, defined, 238
associative property
dividing similar terms, 240–241
of addition and multiplication, 27
evaluate, defined, 236
answers to problems, 35
multiplying similar terms, 240–241
rearranging equations using, 27–29
parentheses in, FOILing, 245–246
parentheses in, removing, 244

• B •
separating into terms, 237–238
simplifying, 242–247

Babylonian numerals, 275
subtracting similar terms, 239
balance scale method for solving
terms, defined, 237
equations, 258
with value given for every variable, 236–237
base of powers, 32
algebraic terms, 238. See also similar terms
base 2 (binary numbers), 277–278
alternative forms of equations
base 16 (hexadecimal numbers), 278
answers to problems, 34–35
binary numbers (base 2), 277–278
associative property for, 27–29
boldfaced text in this book, 2
commutative property for, 24–26, 27–29
borrowing in subtraction, 14, 124, 146
inverse operations for, 24–26
box (rectangular solid), 212–213, 215
amicable numbers, 285
braces ({ }), 57. See also parentheses [( )]

Ancient Greek numerals, 275
brackets ([ ]), 57. See also parentheses [( )]

approximately equals symbol (≈), 30
bundled tally marks, 274
area of a shape
answers to problems, 217–220

• C •
circle, 210, 211
parallelogram, 206, 207
carrying
rectangle, 205–206, 207
in addition, 14, 120–121, 146
rhombus, 206
in multiplication, 15
square, 205, 207

Cartesian graphs
trapezoid, 206–207
answers to problems, 228–232
as Cartesian coordinate system, 221

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Index 289

Cartesian coordinates in, 221
equivalent decimals and fractions (table),
as Cartesian plane, 221

144–145
defined, 221
fractions to decimals, 153–154
drawing lines on, 225–226
fractions to percents, 167–168
function, defined, 225
between improper fractions and mixed
graph, defined, 221
numbers, 94–96
origin, defined, 221
percents to decimals, 163–164
plotting points on, 221–223
percents to fractions, 166
x-axis, defined, 221
reasonability test for, 188
x-coordinate, defined, 221
standard notation to exponential
y-axis, defined, 221
notation, 178
y-coordinate, defined, 221
units of measurement between English and
center of a circle, 210
metric, 193–194
circle, 210–212
cross-multiplying fractions
circumference of a circle, 210, 211
in algebraic equations, 261–262
coefficient of algebraic term, 238
answers to problems, 104–106, 269–270
column lineup
to find common denominator, 100–101
for adding and subtracting decimals, 146
to find the greater, 100–101
for adding and subtracting whole numbers,
when adding, 111–112

14–15
when subtracting, 115
answers to problems, 21
cube, 212–213, 215
combining similar terms, 242–243
cubic numbers, 283
common denominator of fractions
curious types of numbers
adding fractions using, 111–112
amicable numbers, 285
cross-multiplying to find, 100–101
cubic numbers, 283
defined, 111
factorial numbers, 283–284
subtracting fractions using, 115

Fermat primes, 286
subtracting mixed numbers using, 123–126

Mersenne primes, 286
commutative property
perfect numbers, 284–285
of addition and multiplication, 24
powers of two, 284, 286
answers to problems, 34–35
prime numbers, 73–75, 77, 285–286
rearranging equations using, 24–26
square numbers, 32, 281–282
comparing fractions, cross-multiplying for,
triangular numbers, 282

100–101
cylinder, 213–214, 215
composite numbers
answers to problems, 85

• D •
defined, 73
prime numbers versus, 73
decimal number system, 177
tests for, 73–75
decimal numbers
cone, 214, 215
answers to problems, 155–162
constants, 238
converting fractions to, 153–154
continuous versus discrete units, 187
converting percents to, 163–164
converting
converting to fractions, 151–152
decimals to fractions, 151–152
converting to percents, 165
decimals to percents, 165
decimal point for, 141
between English system of measurement, 188
dividing by one another, 149–150

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290 Basic Math & Pre-Algebra Workbook For Dummies
decimal numbers (continued)

decimals by one another, 149–150
equivalent decimals and fractions (table),
decimals by powers of ten, 142

144–145
dividend, defined, 149
expanding as a sum, 141
divisibility tests, 69–71
fractions’ similarity to, 141
divisor, defined, 17, 149

Hindu-Arabic number system, 7–8
fractions (multiplying by reciprocal),
multiplying, 148

109–111
place value in, 141
as inverse of multiplying, 24
repeating decimals, 141, 153–154
long, 17–18
rounding, 142
mixed numbers, 118–120
terminating decimals, 141, 153–154
with negative numbers, 43–44
trailing zero in, 142
number line for, 12, 13
decimal point (.)

order of operations for, 59
for money amounts less than one dollar, 141
powers of ten by one another, 180
moving to multiply or divide by powers of
quotient, defined, 149
ten, 142
for reducing fractions, 97–98
decomposition
remainder, avoiding when dividing deci-
answers to problems, 86
mals, 149–150
defined, 77
remainder, defined, 17
factorization tree for, 77
with scientific notation, 183
for reducing fractions, 97
similar terms, 240–241
denominator of fractions
divisibility. See also factors; multiples
common, adding fractions using, 111–112
answers to problems, 84
common, cross-multiplying for, 100–101
defined, 69
common, defined, 111
as prime number test, 73–74
common, subtracting fractions using, 115
tests for, 69–71
common, subtracting mixed numbers using,
divisor, defined, 17, 149

123–124
doesn’t equal symbol (≠), 30
defined, 91
dot (.) for multiplication, 2, 7, 235
fraction value if 1, 92
drawing lines on Cartesian graphs, 225–226
numerator equal to, 92
diameter of a circle, 210, 211

• E •
digits. See also numbers
answers to problems, 19–20

Egyptian numerals, 274
defined, 7

English system of measurement
identifying, 8–9
for area of a shape, 202
rounding to, 10–11
commonly used units (table), 188
discrete versus continuous units, 187
converting between units, 188
distance measurements
converting to and from metric system,

English system, 188

193–194
metric system, 190
reasonability test for conversions, 188
dividend, defined, 149
equations
dividing. See also arithmetic expressions
alternative forms, 24–26, 27–29
algebra conventions for, 235
associative property for rearranging, 27–29
in balance scale method for solving
balance scale method for solving, 258
equations, 258

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Index 291
commutative property for rearranging,
fluid volume measurements

24–26

English system, 188
cross-multiplying fractions, 261–262
metric system, 190
guessing and checking to solve, 255, 256

FOILing two sets of parentheses, 245–246
inspection for solving, 255, 256
fractions
inverse operations for rearranging, 24–26
adding, 111–114
rearranging to isolate x, 259–260
answers to problems, 102–105, 127–140
rewriting the problem, 255, 256
converting decimals to, 151–152
simplifying expressions before solving, 256
converting percents to, 166
evaluate, defined, 49, 236
converting to decimals, 153–154

Example icon, 4
converting to percents, 167–168
exclamation point (!) as factorial symbol, 283
cross-multiplying, 100–101, 111–112, 115,
expanding a decimal as a sum, 141

261–262
exponent of powers
decimals’ similarity to, 141
defined, 32
decomposition for reducing, 97
multiplying identical variables, 244
denominator of, 91
notation in algebra, 235
dividing (multiplying by reciprocal), 109–111
powers of ten, 178, 180
equivalent decimals and fractions (table),
expressions. See algebraic expressions; arith-

144–145
metic expressions
improper, converting to and from mixed
numbers, 94–96

• F •
improper, defined, 91, 94
increasing terms of, 97
factorial numbers, 283–284
lowest terms for, 97
factorization tree, 77
mixed numbers, 94, 118–126
factors. See also divisibility
multiplying, 107–109
of amicable numbers, 285
with numerator and denominator equal, 92
answers to problems, 84–88
numerator of, 91
in composite numbers, 73
with 1 as denominator, 92
defined, 72
proper, defined, 91, 94
finding all for a number, 75–76
reciprocal, 92, 109–110
greatest common factor (GCF), 79–80
reducing, 97–98, 107–108
multiples of, 72
subtracting, 115–117
of perfect numbers, 284–285
functions, 225–226
prime factors, 77–78
in prime numbers, 73, 285

• G •
feet
converting to and from meters, 193
gallons
converting to other English system units, 188
converting to and from liters, 193

Fermat numbers, 286
converting to other English system units,

Fermat, Pierre de (mathematician), 286

188

Fermat primes, 286

GCF (greatest common factor), 79–80
fill-in-the blank problems
geometry
answers to problems, 34–35
answers to problems, 217–220
solving with alternative forms of equations,
box (rectangular solid), 212–213, 215

24–26

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292 Basic Math & Pre-Algebra Workbook For Dummies
geometry (continued)

inequalities, 30–31
circle, 210–212
input of a function
cone, 214, 215
defined, 225
cube, 212–213, 215
input-output table, 225–226
cylinder, 213–214, 215
inspection method for algebraic equations,
defined, 201

255, 256
parallelogram, 206, 207
inverse operations
perimeter, defined, 201
addition and subtraction, 23–24
prism, 213–214, 215
alternative forms of equations using,
pyramid, 214, 215

24–26
quadrilaterals, 201, 205–209
answers to problems, 34–35
rectangle, 205–206, 207
defined, 23
rhombus, 206
multiplication and division, 24
shapes, defined, 201
for rewriting algebraic equations, 254, 255
solids, defined, 201
squaring, 32
square, 205, 207
italicized text in this book, 2
trapezoid, 206–207
triangles, 201, 202–204

• K •
units of measurement for area, 202
graph, 221. See also Cartesian graphs
kilograms, converting to and from pounds, 193
greater than symbol (>), 30
kilometers, converting to and from miles, 193
greatest common factor (GCF), 79–80
guessing and checking method for algebraic

• L •
equations, 254, 255
leading zeros

• H •
counting for negative numbers, 178
as unnecessary in whole numbers, 8
heptagons, 201
least common multiple (LCM), 82–83
hexadecimal numbers (base 16), 278
legs of a right triangle, 202, 203
hexagons, 201
less than symbol (<), 30

Hindu-Arabic number system
liters, converting to and from gallons, 193
overview, 7–8
long division, 17–18. See also dividing
powers of ten as basis of, 32, 177
lowest terms for fractions, 97
hundreds digit
answers to problems, 19–20

• M •
identifying, 8–9
rounding to, 10–11
mass, 190
hypotenuse of a right triangle, 202, 203

Mayan numerals, 276–277
measurement. See units of measurement

• I •

Mersenne, Marin (mathematician), 286

Mersenne numbers, 286
improper fractions. See also fractions

Mersenne primes, 286
converting to and from mixed numbers,
meters, converting to and from feet, 193

94–96
metric system of measurement
defined, 91
for area of a shape, 202
mixed numbers as alternative form, 94
basic units (table), 190
increasing terms of fractions, 97

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Index 293
converting to and from English system,
powers of ten together, 180

193–194
by reciprocal for fraction division, 109–110
prefixes (table), 191
with scientific notation, 182, 183
miles, converting to and from kilometers, 193
similar terms, 240–241
minus sign (–)

symbols for, 2, 7
adjacent minus signs, 39, 42
combining similar terms with, 242

• N •
for negative numbers, 37
outside absolute value bars, 39
negating a number, 39–40
preceding parentheses, 244
negative numbers
mixed numbers
addition with, 40–41
adding, 120–123
answers to problems, 45–47
converting to and from improper fractions,
defined, 37

94–96
division with, 43–44
defined, 94
multiplication with, 43–44
dividing, 118–120
negating a number, 39–40
equivalent decimals and fractions (table),
number line for, 37–38

144–145
powers of ten with, 178
improper fractions as alternative form, 94
as subtraction result, 37–38
multiplying, 118–120
subtraction with, 42
subtracting, 123–126
nested parentheses, 57–58
mixed-operator expressions, evaluating, 52–53
nonpolygons, 202
multiples. See also factors
number line
answers to problems, 84–85, 87–88
answers to problems, 20–21, 45
defined, 72
for big four, 12–13
generating for a number, 81
for negative numbers, 37–38, 41, 42
least common multiple (LCM), 82–83
numbers. See also curious types of numbers;
multiplying. See also arithmetic expressions
negative numbers; numeral and number
algebra conventions for, 2, 7, 235
systems
associative property, 27
absolute value of, 39
in balance scale method for solving equa-
answers to problems, 19–22
tions, 258
column lineup, 14–15
commutative property, 24
constants as, 238
cross-multiplying fractions, 100–101,
digits, defined, 7

111–112, 115, 261–262
identifying digits, 8–9
decimals by powers of ten, 142
large, long division for, 17–18
decimals together, 148
large, multiplying, 15–16

FOILing two sets of parentheses, 245–246
large, rounding for easier work with, 37–38
fractions, 107–109
large, subtracting from small numbers,
identical variables, 244

37–38
as inverse of dividing, 24
leading zeros, 8, 178
in long division cycle, 17–18
number line, 12–13, 37–38, 41
mixed numbers, 118–120
numerals, defined, 7
by multiple digits, 15–16
numerals versus, 273
with negative numbers, 43–44
place value in, 7–8, 141
number line for, 12, 13
placeholders, zeros as, 8, 177
order of operations for, 59
rounding, 10–11

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294 Basic Math & Pre-Algebra Workbook For Dummies
numeral and number systems
associative property use with, 27–29

Ancient Greek numerals, 275
braces and brackets as forms of, 57

Babylonian numerals, 275
for math group operations, 27
base 2 (binary numbers), 277–278
nested, 57–58
base 16 (hexadecimal numbers), 278
order of operations for, 59
bundled tally marks, 274
when subtracting negative numbers, 42

Egyptian numerals, 274
pentagons, 201

Hindu-Arabic number system, 7–8, 32, 177

Percent Circle, 169–170

Mayan numerals, 276–277
percents
numerals versus numbers, 273
answers to problems, 172–174
prime-based numbers, 279–280
converting decimals to, 165

Roman numerals, 276
converting fractions to, 167–168
tally marks, 273
converting to decimals, 163–164
numerals. See also numbers; numeral and
converting to fractions, 166
number systems

Percent Circle for calculating, 169–170
defined, 7
percent, defined, 163
numbers versus, 273
perfect numbers, 284–285
numerator of fractions, 91, 92
perimeter of a shape
circle circumference, 210, 211

• O •
defined, 201
rectangle, 205–206, 207
octagons, 201
square, 205, 207
one (1)

place value
as fraction denominator, 92
in decimal numbers, 141
fractions equal to, 92
in whole numbers, 7–8
as neither prime nor composite, 73
placeholders
rules for powers containing, 32
scientific notation for, 177
ones digit, identifying, 8–9
zeros as, 8, 177
order of operations, 49, 59
plotting points on Cartesian graphs,
origin of Cartesian graphs, 221

221–223
output of a function
plus sign (+)

defined, 225
combining similar terms with, 242
input-output table, 225–226
preceding parentheses, 244
polygons

• P •
answers to problems, 217–219
classification by number of sides
parallelogram, 206, 207

(table), 201
parentheses [( )]

defined, 201
in algebraic expressions, FOILing, 245–246
parallelogram, 206, 207
in algebraic expressions, removing, 244
quadrilaterals, 201, 205–209
answers to problems, 35, 61–67
rectangle, 205–206, 207
in arithmetic expressions with powers,
rhombus, 206

56–57
square, 205, 207
in arithmetic expressions without powers,
trapezoid, 206–207

54–55
triangles, 201, 202–204

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Index 295
pounds
proper fractions. See also fractions
converting to and from kilograms, 193
defined, 91
converting to other English system
multiplying, 107
units, 188
pyramid, 214, 215
powers. See also powers of ten

Pythagorean theorem, 202–203
algebra conventions for, 235
answers to problems, 36

• Q •
in arithmetic expressions with parentheses,

56–57
quadrilaterals
in arithmetic expressions without parenthe-
answers to problems, 218–219
ses, 53–54
defined, 205
base of, 32
number of sides, 201
containing 0 or 1, rules for finding, 32
parallelogram, 206, 207
defined, 32
rectangle, 205–206, 207
exponent of, 32
rhombus, 206

Fermat numbers based on, 286
square, 205, 207

Mersenne numbers based on, 286
trapezoid, 206–207
order of operations for, 59
quotient, defined, 149
square numbers, 32, 281–282
of two, 284, 286

• R •
powers of ten
calculating, for positive whole numbers, 32
radius of a circle, 210
defined, 32, 177
rate measurement (English system), 188
dividing by one another, 180
rearranging equations to isolate x, 259–260
dividing decimals by, 142
reasonability test for conversions between
exponential notation for, 178

English system units, 188

Hindu-Arabic number system based on,
reciprocal of a fraction

32, 177
defined, 92
identifying, 178
for fraction division, 109–110
multiplying decimals by, 142
rectangle, 205–206, 207
multiplying together, 180
rectangular solid (box), 212–213, 215
negative numbers with, 178
reducing fractions
standard notation for, 178
decomposition for, 97
prefixes in metric system (table), 191
dividing for, 97–98
prime factors, 77–78
when multiplying, 107–108
prime numbers
remainder
answers to problems, 85
avoiding when dividing decimals, 149–150
composite numbers versus, 73
defined, 17
defined, 73, 285

Remember icon, 4

Fermat, 286
removing parentheses in algebraic expres-

Mersenne, 286
sions, 244
as prime factors, 77
repeating decimals
tests for, 73–75
converting fractions to, 153–154
prime-based numbers, 279–280
defined, 141
prism, 213–214, 215
rewriting algebraic equations, 254, 255

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296 Basic Math & Pre-Algebra Workbook For Dummies
rhombus, 206
simplifying algebraic equations
right triangles, 202–203
balance scale method, 258

Roman numerals, 276
cross-multiplying fractions, 261–262
rounding numbers
rearranging to isolate x, 259–260
answers to problems, 19–20, 155
rewriting the problem, 255, 256
to avoid remainder when dividing decimals,
simplifying algebraic expressions

149–150
combining similar terms, 242–243
decimals, 142

FOILing two sets of parentheses, 245–246
overview and practice, 10–11
removing parentheses, 244
before solving equations, 256

• S •
sixteen (16), as base of hexadecimal number
system, 278
scientific notation
solids. See also volume of a solid
answers to problems, 184–186
box (rectangular solid), 212–213, 215
described, 177, 181
cone, 214, 215
multiplying and dividing with, 182–183
cube, 212–213, 215
writing numbers in, 181
cylinder, 213–214, 215
shapes. See also area of a shape; perimeter of
defined, 201
a shape
prism, 213–214, 215
circle, 210–212
pyramid, 214, 215
cone, 214, 215
solving algebraic equations
defined, 201
answers to problems, 264–270
measurements for area of, 202
balance scale method, 258
nonpolygons, 202
cross-multiplying fractions, 261–262
parallelogram, 206, 207
defined, 255
polygons, 201
by guessing and checking, 255, 256
quadrilaterals, 201, 205–209
by inspection, 255, 256
rectangle, 205–206, 207
rearranging equations to isolate x,
rhombus, 206

259–260
square, 205, 207
by rewriting the problem, 255, 256
trapezoid, 206–207
simplifying expressions first, 256
triangles, 201, 202–204
speed measurement (English system), 188
sign of numbers. See also negative numbers
square, 205, 207
affect of absolute value on, 39
square numbers, 32, 281–282
affect of negation on, 39
square root, 32
affect on multiplication and division
subtracting. See also arithmetic
results, 43
expressions
preceding parentheses, 244
in balance scale method for solving equa-
when combining similar terms, 242
tions, 258
similar terms
borrowing, 14, 124
adding and subtracting, 239
column lineup for, 14, 15, 146
combining, 242–243
decimal numbers, 146
defined, 238
fractions, 115–117
multiplying and dividing, 240–241
as inverse of adding, 23–24

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Index 297
large number from small number, 37–38
triangles
in long division cycle, 17–18
answers to problems, 217–218
mixed numbers, 123–126
area, 202, 203
with negative numbers, 42
defined, 201, 202
number line for, 12–13

Pythagorean theorem, 202–203
order of operations for, 59
right, 202–203
similar terms, 239
triangular numbers, 282
trigonometry, 202

• T •
two (2)

as base of binary numbers, 277–278
tally marks, 273–274
divisibility test for, 70
temperature measurements

Fermat numbers based on powers of, 286

English system, 188

Mersenne numbers based on powers
metric system, 190
of, 286
ten, powers of. See powers of ten
powers of, 284, 286
tens digit
answers to problems, 19–20

• U •
identifying, 8–9
rounding to, 10–11
units of measurement
terminating decimals
answers to problems, 195–199
converting fractions to, 153–154
for area of a shape, 202
defined, 141
continuous versus discrete units, 187
terms. See also similar terms
converting between English and metric,
algebraic, 238

193–194
constants, 238

English system, 187–190
defined, 237
metric system, 190–192
thousands digit
need for, 187
answers to problems, 19–20
identifying, 8–9

• V •
rounding to, 10–11
time measurements
variables

English system, 188
in algebraic terms, 238
metric system, 190
conventions in this book for, 2
times sign (×), dot used instead of,
defined, 235

2, 7, 235
identical, multiplying, 244

Tip icon, 4
volume, fluid, 188, 190
trailing zeros
volume of a solid
for converting fraction to exact decimal
answers to problems, 220
value, 154
box (rectangular solid), 212–213, 215
counting when multiplying decimals, 148
cone, 214, 215
defined, 142
cube, 212–213, 215
for dividing decimals, 149–150
cylinder, 213–214, 215
trapezoid, 206–207
prism, 213–214, 215
pyramid, 214, 215

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298 Basic Math & Pre-Algebra Workbook For Dummies

• W •

• Z •

Warning! icon, 4
zero
weight measurements
absolute value of, 39

English system, 188
leading, 8, 178
metric system (mass), 190
as placeholder, 8, 177
rules for powers containing, 32

• X •
trailing, 142, 148, 149–150, 154
x-axis, 221
x-coordinate, 221

• Y •
y-axis, 221
y-coordinate, 221

Document Outline

Basic Math & Pre-Algebra Workbook for Dummies

Contents at a Glance

Table of Contents

Introduction

- About This Book
- Conventions Used in This Book
- Foolish Assumptions
- How This Book Is Organized
- Icons Used in This Book
- Where to Go from Here

Part I: Back to Basics with Basic Math

- Chapter 1: We’ve Got Your Numbers

- Getting in Place with Numbers and Digits
- Rollover: Rounding Numbers Up and Down
- Using the Number Line with the Big Four
- The Column Lineup: Adding and Subtracting
- Multiplying Multiple Digits
- Cycling through Long Division
- Solutions to We’ve Got Your Numbers

- Chapter 2: Smooth Operators: Working with the Big Four Operations

- Switching Things Up with Inverse Operations and the Commutative Property

- Getting with the In-Group: Parentheses and the Associative Property

- Becoming Unbalanced: Inequalities
- Special Times: Powers and Square Roots
- Answers to Problems in Smooth Operators

- Chapter 3: Getting Down with Negative Numbers

- Understanding Where Negative Numbers Come From

- Sign-Switching: Understanding Negation and Absolute Value

- Adding with Negative Numbers

- Subtracting with Negative Numbers

- Comparing Signs of the Times( And Division) of Negative Numbers

- Answers to Problems in Getting Down with Negative Numbers

- Chapter 4: It’s Just an Expression

- Evaluating Expressions with Addition and Subtraction

- Evaluating Expressions with Multiplication and Division

- Making Sense of Mixed-Operator Expressions
- Handling Powers Responsibly
- Prioritizing Parentheses
- Pulling Apart Parentheses and Powers
- Figuring Out Nested Parentheses

- Bringing It All Together: The Order of Operations

- Solutions to It’s Just an Expression

- Chapter 5: Dividing Attention: Divisibility, Factors, and Multiples

- Checking for Leftovers: Divisibility Tests

- Understanding Factors and Multiples

- One Number, Indivisible: Identifying Prime (And Composite) Numbers

- Generating a Number’s Factors
- Decomposing a Number into Its Prime Factors
- Finding the Greatest Common Factor (GCF)
- Generating the Multiples of a Number
- Finding the Least Common Multiple (LCM)

- Solutions to Divisibility, Factors, and Multiples

Part II: Slicing Things Up: Fractions, Decimals, and Percents

- Chapter 6: Fractions Are a Piece of Cake

- Getting Down the Basic Fraction Stuff

- In Mixed Company: Converting between Mixed Numbers and Improper Fractions

- Increasing and Reducing the Terms of Fractions

- Comparing Fractions with Cross-Multiplication

- Solutions to Fractions Are a Piece of Cake

- Chapter 7: Fractions and the Big Four

- Multiplying Fractions: A Straight Shot

- Flipping for Fraction Division

- Reaching the Common Denominator: Adding Fractions

- The Other Common Denominator: Subtracting Fractions

- Multiplying and Dividing Mixed Numbers

- Carried Away: Adding Mixed Numbers

- Borrowing from the Whole: Subtracting Mixed Numbers

- Solutions to Fractions and the Big Four

- Chapter 8: Getting to the Point with Decimals

- Getting in Place: Basic Decimal Stuff

- Knowing Simple Decimal-Fraction Conversions

- A New Lineup: Adding and Subtracting Decimals

- Counting Decimal Places: Multiplying Decimals

- Decimal Points on the Move: Dividing Decimals

- Changing Decimals to Fractions

- Changing Fractions to Decimals

- Solutions to Getting to the Point with Decimals

- Chapter 9: Playing the Percentages

- Converting Percents to Decimals
- Changing Decimals to Percents
- Switching from Percents to Fractions
- Converting Fractions to Percents
- Mixing Things Up with the Percent Circle
- Solutions to Playing the Percentages

Part III: A Giant Step Forward: Intermediate Topics

- Chapter 10: Seeking a Higher Power through Scientific Notation

- On the Count of Zero: Understanding Powers of Ten

- Exponential Arithmetic: Multiplying and Dividing Powers of Ten

- Representing Numbers in Scientific Notation

- Multiplying and Dividing with Scientific Notation

- Answers to Problems in Seeking a Higher Power through Scientific Notation

- Chapter 11: Weighty Questions on Weights and Measures

- The Basics of the English System
- Going International with the Metric System
- Converting Between English and Metric Units

- Answers to Problems in Weighty Questions on Weights and Measures

- Chapter 12: Shaping Up with Geometry

- Getting in Shape: Polygon (And Nonpolygon) Basics

- Making a Triple Play with Triangles
- Taking a Fourth Side with Quadrilaterals
- Getting Around with Circle Measurements
- Building Solid Measurement Skills

- Answers to Problems in Shaping Up with Geometry

- Chapter 13: Getting Graphic: Cartesian Graphs

- Getting the Point of the Cartesian Graph

- Drawing the Line on the Cartesian Graph

- Answers to Problems in Getting Graphic: Cartesian Graphs

Part IV: The X Factor: Introducing Algebra

- Chapter 14: Expressing Yourself with Algebraic Expressions

- Plug It In: Evaluating Algebraic Expressions

- Knowing the Terms of Separation
- Adding and Subtracting Similar Terms
- Multiplying and Dividing Terms

- Simplifying Expressions by Combining Similar Terms

- Simplifying Expressions with Parentheses

- FOILing: Dealing with Two Sets of Parentheses

- Answers to Problems in Expressing Yourself with Algebraic Expressions

- Chapter 15: Finding the Right Balance: Solving Algebraic Equations

- Solving Simple Algebraic Equations

- Equality for All: Using the Balance Scale to Isolate x

- Switching Sides: Rearranging Equations to Isolate x

- Barring Fractions: Cross-Multiplying to Simplify Equations

- Answers to Problems in Finding the Right Balance: Solving Algebraic Equations

Part V: The Part of Tens

- Chapter 16: Ten Alternative Numeral and Number Systems

- Tally Marks
- Bundled Tally Marks
- Egyptian Numerals
- Babylonian Numerals
- Ancient Greek Numerals
- Roman Numerals
- Mayan Numerals
- Base 2, or Binary Numbers
- Base 16, or Hexadecimal Numbers
- Prime-Based Numbers

- Chapter 17: Ten Curious Types of Numbers

- Square Numbers
- Triangular Numbers
- Cubic Numbers
- Factorial Numbers
- Powers of Two
- Perfect Numbers
- Amicable Numbers
- Prime Numbers
- Mersenne Primes
- Fermat Primes

Index

Table of Contents

- Basic Math & Pre-Algebra Workbook for Dummies

- Contents at a Glance
- Table of Contents
- Introduction
- Conventions Used in This Book
- Icons Used in This Book
- Part I: Back to Basics with Basic Math
- Chapter 1: We’ve Got Your Numbers
- Rollover: Rounding Numbers Up and Down
- Using the Number Line with the Big Four
- The Column Lineup: Adding and Subtracting
- Multiplying Multiple Digits
- Cycling through Long Division
- Solutions to We’ve Got Your Numbers

- Chapter 2: Smooth Operators: Working with the Big Four Operations

- Getting with the In-Group: Parentheses and the Associative Property

- Becoming Unbalanced: Inequalities
- Special Times: Powers and Square Roots
- Answers to Problems in Smooth Operators

- Chapter 3: Getting Down with Negative Numbers

- Sign-Switching: Understanding Negation and Absolute Value

- Adding with Negative Numbers

- Subtracting with Negative Numbers

- Comparing Signs of the Times( And Division) of Negative Numbers

- Answers to Problems in Getting Down with Negative Numbers

- Chapter 4: It’s Just an Expression

- Evaluating Expressions with Multiplication and Division

- Making Sense of Mixed-Operator Expressions
- Handling Powers Responsibly
- Prioritizing Parentheses
- Pulling Apart Parentheses and Powers
- Figuring Out Nested Parentheses

- Bringing It All Together: The Order of Operations

- Solutions to It’s Just an Expression

- Chapter 5: Dividing Attention: Divisibility, Factors, and Multiples

- Understanding Factors and Multiples

- One Number, Indivisible: Identifying Prime (And Composite) Numbers

- Generating a Number’s Factors
- Decomposing a Number into Its Prime Factors
- Finding the Greatest Common Factor (GCF)
- Generating the Multiples of a Number
- Finding the Least Common Multiple (LCM)

- Solutions to Divisibility, Factors, and Multiples

- Part II: Slicing Things Up: Fractions, Decimals, and Percents

- Chapter 6: Fractions Are a Piece of Cake

- In Mixed Company: Converting between Mixed Numbers and Improper Fractions

- Increasing and Reducing the Terms of Fractions

- Comparing Fractions with Cross-Multiplication

- Solutions to Fractions Are a Piece of Cake

- Chapter 7: Fractions and the Big Four