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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Serie: Wiley Self-Teaching Guides
- Sprache: Englisch
A comprehensive and hands-on guide to crucial math concepts and terminology In the newly revised third edition of All the Math You'll Ever Need: A Self-Teaching Guide, veteran math and computer technology teacher Carolyn Wheater and veteran mathematics author Steve Slavin deliver a practical and accessible guide to math you can use every day and apply to a wide variety of life tasks. From calculating monthly mortgage payments to the time you'll need to pay off a credit card, this book walks you through the steps to understanding basic math concepts. This latest edition is updated to reflect recent changes in interest rates, prices, and wages, and incorporates information on the intelligent and efficient use of calculators and mental math techniques. It also offers: * A brand-new chapter on hands-on statistics to help readers understand common graphs * An easy-to-use-format that provides an interactive method with frequent questions, problems, and self-tests * Complete explanations of necessary mathematical concepts that explore not just how math works, but also why it works Perfect for anyone seeking to make practical use of essential math concepts and strategies in their day-to-day life, All the Math You'll Ever Need is an invaluable addition to the libraries of students who want a bit of extra help applying math in the real world.
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Veröffentlichungsjahr: 2022
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Table of Contents
Cover
Title Page
Copyright
How to Use This Book
Acknowledgments
1 Getting Started
2 Essential Arithmetic
1 ADDITION
2 MULTIPLICATION
3 SUBTRACTION
4 DIVISION
3 Focus on Multiplication
1 THE MULTIPLICATION TABLE
2 LEARNING THE MULTIPLICATION TABLE
3 TESTING YOUR KNOWLEDGE
4 ABOUT LONG MULTIPLICATION
5 PRACTICE EXERCISES WITH TWO DIGITS
6 MULTIPLYING THREE DIGITS
7 FOUR-DIGIT MULTIPLICATION
8 WORD PROBLEMS WITH MULTIPLICATION
4 Focus on Division
1 SHORT DIVISION
2 LONG DIVISION
3 WORD PROBLEMS WITH DIVISION
5 Mental Math
1 FAST MULTIPLICATION
2 FAST DIVISION
3 HOW ARE YOU DOING SO FAR?
4 MENTAL MATH TRICKS
6 Positive and Negative Numbers
1 WHAT ARE NEGATIVE NUMBERS?
2 ADDING POSITIVE AND NEGATIVE NUMBERS
3 SUBTRACTING POSITIVE AND NEGATIVE NUMBERS
4 MULTIPLYING POSITIVE AND NEGATIVE NUMBERS
5 DIVISION WITH POSITIVE AND NEGATIVE NUMBERS
7 Fractions
1 CHANGING THE LOOK OF FRACTIONS
2 ADDING FRACTIONS
3 SUBTRACTING FRACTIONS
4 MULTIPLYING BY FRACTIONS
5 DIVIDING BY FRACTIONS
6 CANCELLING
8 Decimals
1 ADDING AND SUBTRACTING WITH DECIMALS
2 MULTIPLYING WITH DECIMALS
3 DIVIDING WITH DECIMALS
9 Converting Fractions and Decimals
1 CONVERTING FRACTIONS INTO DECIMALS
2 CONVERTING DECIMALS INTO FRACTIONS
3 SIMPLIFYING FRACTIONS
4 REPEATING DECIMALS
5 NON-TERMINATING DECIMALS
10 Ratios and Proportions
1 RATIOS
2 PROPORTIONS
11 Solving Problems
1 WORDS AND FRACTIONS
2 WORD PROBLEMS WITH MONEY
12 Area and Perimeter
1 AREAS OF RECTANGLES
2 WHEN WE USE AREA
3 PERIMETERS OF RECTANGLES
4 AREAS OF TRIANGLES
13 Circumference and Area
1 CIRCUMFERENCE OF A CIRCLE
2 THE AREA OF A CIRCLE
14 Percentages
1 FRACTIONS, DECIMALS, AND PERCENTS
2 PERCENT CHANGE
3 FAST PERCENT CHANGE
4 PERCENT DISTRIBUTION
5 TIPPING
15 Solving Simple Equations
1 THE CARE AND TREATMENT OF EQUATIONS
2 ISOLATING
X
3 ADDITION AND SUBTRACTION WITH
X
4 MULTIPLICATION AND DIVISION WITH
X
5 FRACTIONS AND DECIMALS IN EQUATIONS
6 NEGATIVE NUMBERS
7 MULTI-STEP EQUATIONS
16 Powers and Roots
1 EXPONENTS
2 ROOTS
3 ADVANCED PROBLEMS
17 Very Large and Very Small Numbers
1 WRITING LARGE NUMBERS
2 SCIENTIFIC NOTATION
3 MILLIONS, BILLIONS, TRILLIONS, AND BEYOND
4 MULTIPLYING LARGE NUMBERS
5 DIVIDING LARGE NUMBERS
6 SCIENTIFIC NOTATION FOR VERY SMALL NUMBERS
18 Algebra Problems
1 AGE PROBLEMS
2 FINDING THE NUMBERS
3 MIXTURE PROBLEMS
19 Interest Rates
1 SIMPLE INTEREST
2 COMPOUND INTEREST
3 A WORD ON CONTINUOUS COMPOUNDING
4 DOUBLING TIME: THE RULE OF 70
5 DISCOUNTING
20 Rate, Time, and Distance
1 THE MAGIC FORMULA
2 THE TERMS
3 FINDING DISTANCE
4 FINDING RATE
5 FINDING TIME
6 RATE, TIME, AND DISTANCE PROBLEMS
7 SPEED LIMIT PROBLEMS
21 Personal Finance
1 MARK-DOWN PROBLEMS
2 SALES TAX PROBLEMS
3 CREDIT CARDS
4 FEDERAL INCOME TAX
5 MORTGAGE INTEREST AND TAXES
22 Business Math
1 COMMISSIONS
2 MARK-UPS
3 DISCOUNTING FROM LIST PRICE
4 QUANTITY DISCOUNTS
5 2/10 N/30
6 CHAIN DISCOUNTS
7 PROFIT
23 A Taste of Statistics
1 DISTRIBUTIONS OF DATA
2 MEASURES OF CENTER
3 MEASURES OF SPREAD
4 ESTIMATING
24 Review
CHAPTER 3 REVIEW
CHAPTER 4 REVIEW
CHAPTER 5 REVIEW
CHAPTER 6 REVIEW
CHAPTER 7 REVIEW
CHAPTER 8 REVIEW
CHAPTER 9 REVIEW
CHAPTER 10 REVIEW
CHAPTER 11 REVIEW
CHAPTER 12 REVIEW
CHAPTER 13 REVIEW
CHAPTER 14 REVIEW
CHAPTER 15 REVIEW
CHAPTER 16 REVIEW
CHAPTER 17 REVIEW
CHAPTER 18 REVIEW
CHAPTER 19 REVIEW
CHAPTER 20 REVIEW
CHAPTER 21 REVIEW
CHAPTER 22 REVIEW
CHAPTER 23 REVIEW
ANSWERS FOR CHAPTER 3 REVIEW
ANSWERS FOR CHAPTER 4 REVIEW
ANSWERS FOR CHAPTER 5 REVIEW
ANSWERS FOR CHAPTER 6 REVIEW
ANSWERS FOR CHAPTER 7 REVIEW
ANSWERS FOR CHAPTER 8 REVIEW
ANSWERS FOR CHAPTER 9 REVIEW
ANSWERS FOR CHAPTER 10 REVIEW
ANSWERS FOR CHAPTER 11 REVIEW
ANSWERS FOR CHAPTER 12 REVIEW
ANSWERS FOR CHAPTER 13 REVIEW
ANSWERS FOR CHAPTER 14 REVIEW
ANSWERS FOR CHAPTER 15 REVIEW
ANSWERS FOR CHAPTER 16 REVIEW
ANSWERS FOR CHAPTER 17 REVIEW
ANSWERS FOR CHAPTER 18 REVIEW
ANSWERS FOR CHAPTER 19 REVIEW
ANSWERS FOR CHAPTER 20 REVIEW
ANSWERS FOR CHAPTER 21 REVIEW
ANSWERS FOR CHAPTER 22 REVIEW
ANSWERS FOR CHAPTER 23 REVIEW
WHERE TO FROM HERE?
Index
End User License Agreement
List of Tables
Chapter 3
Table 3.1 Blank Multiplication Table
Table 3.2 Completed Multiplication Table
Table 3.3 Another Blank Multiplication Table
Chapter 5
Table 5.1 Divisibility Rules
Guide
Cover
Table of Contents
Title Page
Copyright
How to Use This Book
Acknowledgments
Begin Reading
Index
End User License Agreement
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Wiley Self‐Teaching Guides teach practical skills in mathematics and science. Look for them at your local bookstore.
Other Science and Math Wiley Self‐Teaching Guides:
Science
Basic Physics: A Self‐Teaching Guide, Third Edition, by Karl F. Kuhn and Frank Noschese
Biology: A Self‐Teaching Guide, Third Edition, by Steven D. Garber
Chemistry: A Self‐Teaching Guide, Third Edition, by Clifford C. Houk, Richard Post, and Chad A. Snyder
Math
All the Math You'll Ever Need: A Self‐Teaching Guide, by Steve Slavin
Practical Algebra: A Self‐Teaching Guide, Second Edition, by Peter H. Selby and Steve Slavin
Quick Algebra Review: A Self‐Teaching Guide, by Peter H. Selby and Steve Slavin
Quick Business Math: A Self‐Teaching Guide, by Steve Slavin
Quick Calculus: A Self‐Teaching Guide, Second Edition, by Daniel Kleppner and Norman Ramsey
All the Math You’ll Ever Need
A Self-Teaching Guide
Third Edition
Carolyn C. Wheater
Steve Slavin
Copyright © 2022 John Wiley & Sons. All rights reserved.
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Library of Congress Cataloging-in-Publication Data is Available:
ISBN 9781119719182 (paperback)ISBN 9781119719229 (epdf)ISBN 9781119719199 (ebook)
Cover Design: Paul McCarthyCover Image: © Frankramspott/Getty Images
How to Use This Book
This book is organized by chapter with periodic self-tests throughout each chapter. Their purpose is to make sure you comprehend material before moving on. If you find that you have made an error, look back at the preceding material to make sure you understand the correct answer. The information is arranged so that it builds on what comes before. To fully understand the information at the end of a chapter, you must first have completed all of the preceding self-tests.
The format of this book lends itself to proper pacing. When you're going too slowly, you'll say to yourself, “This stuff is so easy—I'm getting bored.” You'll be able to skip a few sections and move on to new material. But when you find yourself pounding your fists against the wall and despairing of ever learning math, that may mean you've been moving ahead a bit too quickly.
If you feel that you don't need to read a particular chapter, you may want to take the self-tests anyway. These provide not only a quick review of the subject matter covered in the chapter, but also a good way of gauging what you already know.
Should you find, on the other hand, that you're having trouble doing a certain type of problem, it will be made clear to you that you need to review an earlier section. For example, no one can do simple division without knowing the multiplication table, so everyone who gets stuck at this point will be sent back to learn that table once and for all. Once that's accomplished, it will be clear sailing through the next few chapters.
This book provides a fast-paced review of arithmetic and elementary algebra, with a smattering of statistics thrown in. It is intended to refresh the memory of the high school or college graduate.
The main emphasis here is on getting you to rely on your own mathematical skills. No longer will you be intimidated trying to calculate tips. No longer will you need to whip out your pocket calculator to do simple arithmetic. And you won't have to wait months to see tangible results. You won't even have to wait weeks. In just a few days your friends and colleagues will notice your new mathematical muscles. So don't delay another minute. Turn to Chapter 2 and just watch those brain cells start to grow.
Acknowledgments
From Steve: Many thanks are due, so I'd like to name names. My longtime editor at Wiley, Judith McCarthy, made hundreds of suggestions to improve and update the book. Authors often hate to change even a word, but Judith's editing has made this second edition a much smoother read. Claire McKean did a thorough copyedit, catching dozens of errors that made it through the first edition, and Benjamin Hamilton supervised the production of the book from copyediting through page proofs.
I owe a large debt of gratitude to my family, especially to my nephews, Jonah and Eric Zimiles. Jonah provided me with a blow-by-blow critique of the strengths and weaknesses of my previous book, Economics: A Self-Teaching Guide (Wiley, 1988), on which I was able to build while writing this book. And Eric, after having read that book, recognized its format lent itself best to my writing style and encouraged me to write another book. Eric's daughters, Eleni, 11, Justine, 7, and Sophie, 5, have contributed to the new edition by helping me with my math whenever I happened to get stuck.
My father, Jack, a retired math teacher, provided inspiration of another kind. As the oldest living academic perfectionist, he upholds such unattainable standards that one cannot help but feel tolerance for one's own shortcomings and those of just about everyone else. And finally, I wish to thank my sister, Leontine Temsky, for her rationality and common sense in the most uncommon and irrational of times.
From Carolyn: Every opportunity to work with the incredible folks at Wiley has been a pleasure. This project was no exception. My thanks to go Christine O'Connor, Tom Dinse, and Riley Harding for providing the vision for the project, giving me the freedom to make it my own, and guiding me through every step. I'm grateful also to copyeditor Julie Kerr, whose keen eye and infinite patience make the final product so much better.
1Getting Started
Far too many Americans are mathematically illiterate. Although many of these people are college graduates, they have trouble doing simple arithmetic. One cannot help but wonder how so many people managed to get so far without having mastered basic arithmetic. Math phobia seems to have become fashionable. People who would never think it amusing to claim not to be able to read or write chuckle as they announce, “I can't do math.”
We all have to deal with numbers sometime—in banking, on taxes, in choosing a mortgage. Like it or not, numbers are an important part of our lives, and the importance of numerical literacy is increasing in finance, economics, science, government, and more. It is time math stopped intimidating us.
What we'll be doing in this book is going back to basics. We'll focus on the multiplication table. You'll need to memorize it. If you need an even more basic text, you can refer to one such as Quick Arithmetic: A Self-Teaching Guide, 3rd edition by Robert A. Carmen and Marilyn J. Carmen (Wiley, 2001).
In All the Math You'll Ever Need, the use of complex formulas is generally avoided. Although such formulas have an honored place in mathematics, they rarely need to be memorized. The ones that are used frequently work their way into memory. The others can be looked up when they're needed.
Finally, the use of technical terms is minimized whenever possible. Having the vocabulary to describe mathematical ideas and operations accurately is important to learning but you don't need a lot of fancy language for that. There are no quadratic formulas, logarithmic tables, integrals, or derivatives, and there are only a handful of very simple graphs.
This book was designed to be explored without ever using a calculator or computer. Don't get nervous. You will not be asked to throw away your calculator. Just put it in a safe place for now, to be taken out and used only on proper occasions. A calculator is most effectively used for three tasks: (1) to do calculations that need to be done rapidly, (2) to do repetitive calculations, and (3) to do sophisticated calculations that would take a great deal of time to do without a calculator. Calculators and computers are fast and as accurate as their users allow them to be. Typos are a thing, even on calculators. You need to first know what you want to ask the calculator to do, and then have enough math knowledge to decide if the answer it gives you makes sense.
The trick is to use our calculators for these specific tasks and not for arithmetic functions that we can do in our heads. So put away your calculator and start using your innate mathematical ability.
2Essential Arithmetic
Every number system (and, yes, there are or have been others) is made up of a set of symbols that we call numbers and one or more operations you can perform with them. Those operations make up what we call arithmetic. The basic operation in our number system is addition, the act of putting together. The other operations—multiplication, subtraction, division—are related to, or built from, addition.
1 ADDITION
Addition is, at its heart, about counting. If you have 6 pair of shoes and you buy 3 new pairs, counting will tell you that you now have 9 pairs. You added 6 + 3 and got an answer of 9 by counting. After a while you don't have to count every time, because you get to know that 6 + 3 = 9.
You store a lot of addition facts like that in your memory, but there's a limit to how much memorization can help. You probably know that 4 + 8 = 12, but you're unlikely to memorize the answer to 5,387 + 9,748. Adding larger numbers requires a little more information about our number system.
Place Value
Our number system is a place value system, meaning that the value of a numeral depends on the place it sits in. In the number 444 each 4 has a different meaning. The 4 on the right is in the ones place so it represents 4 ones or simply 4. The 4 on the left is in the hundreds place and represents 4 hundreds or 400. The middle 4 is in the tens place so it represents 4 tens or 40. The number 444 is a shorthand for 400 + 40 + 4.
That expanded form, 400 + 40 + 4, helps to explain how we add large numbers. We add the ones to the ones, the tens to the tens, the hundreds to the hundreds and on up in the place value system. If you need to add 444 + 312, think:
Add the 4 ones and the 2 ones to get 6 ones, the 4 tens with 1 ten to get 5 tens and the 4 hundreds with 3 hundreds to get 7 hundreds. Now that would look like this:
You're probably thinking that you could just write the numbers underneath one another in standard form and add down the columns, and you'd be absolutely correct.
The reason to think about it in expanded form, at least for a few minutes, comes up when you have to add something like 756 + 968. The basic rule is the same.
But you can't squeeze 16 (or 11 or 14) into one place. 756 + 968 does not equal 161114. You've got to do some regrouping, or what's commonly called carrying. Those 14 ones equal 1 ten and 4 ones. You're going to keep the 4 ones in the ones place and move the ten over to the middle place with the rest of the tens. That will turn
You'll do the same sort of regrouping with the 12 tens. Ten of those tens make 1 hundred, leaving 2 tens in the tens place. You can do this without using the expanded form. Add 6 + 8 to get 14. Put down the 4 and carry the one ten.
Add 1 + 5 + 6 to get 12. Put down the 2 (tens) and carry the 1 (hundred).
Add 1 + 7 + 9 to get 17. The 7 goes in the hundreds place and the 1 (thousand) slides into the thousands place.
Problem 1:
Add 312 and 423.
Solution:
All that's necessary is adding the digits in each column: 2 + 3 = 5, 1 + 2 = 3, and 3 + 4 = 7.
Problem 2:
What is the result when 459 is added to 1,276?
Solution:
This one requires a little bit of regrouping. Add 6 + 9 to get 15, put down the 5 and carry 1 to the next column. Then 7 + 5 is 12, plus the 1 you carried is 13. Put down the 3 and carry the 1. You can think of the rest as 2 + 4 + 1 = 7 and the 1 thousand comes down unchanged, or you can think of it as 12 + 4 is 16, plus 1 you carried is 17.
Problem 3:
What is the combined total of 9,671 and 2,859?
Solution:
Here again you're regrouping. In the ones column, 1 + 9 is 10, so put down the 0 and carry the 1. Then 7 + 5 is 12 plus 1 you carried makes 13. Put down the 3 and carry the 1. Add 6 + 8 + 1 to get 15. Put down the 5 and carry the 1. Finally, 9 + 2 + 1 is 12.
2 MULTIPLICATION
Multiplication is repeated addition. For instance, you probably know 4 × 3 is 12 because you searched your memory for that multiplication fact. There's nothing wrong with that.
Another way to calculate 4 × 3 is to think of it as adding four threes, or adding three fours.
What about 5 × 7? Maybe you know it's 35, but you could always do this:
You do multiplication instead of addition because it's shorter—sometimes much shorter. Suppose you needed to multiply 78 × 95. If you set this up as an addition problem, you'd have to write 78 copies of 95 before you could even start adding.
Let's set this up as a regular multiplication problem and take a look at the expanded form.
The key to this multiplication is you have to multiply 8 × 5 and 8 × 90 and then multiply 70 × 5 and 70 × 90, and add up all the results. Don't get discouraged, because there is a condensed form.
The first set of numbers we'd multiply would be 8 × 5. You probably know, or can figure out, that's 40. (We'll focus on all the multiplication facts you should memorize in Chapter 3, “Focus on Multiplication.”) Then we'd multiply 8 × 90, which just means multiplying 8 × 9 and putting a zero at the end. Whenever you multiply a number that ends in zero, you can deal with the non-zero parts and add the zero at the end. (See Chapter 5, “Mental Math” for more on that shortcut.) 8 × 9 =72 so 8 × 90 = 720. Next would come 70 × 5. 7 × 5 = 35 so 70 × 5 =350. The last multiplication would be 70 × 90. Multiply 7 × 9 = 63, and then add a zero for the 70 and another zero for the 90. 70 × 90 = 6,300. Add up 6,300 + 350 + 720 + 40 to get 7,410.
Here's how to write it more compactly. Multiply 8 × 5 = 40, put down the 0 and carry the 4. 8 × 9 = 72 and the 4 we carried makes 76. Write the 76 in front of that 0 you put down and you see 760. This 760 is the 40 and the 720 combined. Now, you need to multiply 95 by 70, which means multiply by 7 and add a zero. So put the zero down first, under the 0 of the 760. Then 7 × 5 = 35. Put down the 5 to the left of the 0 and carry the 3. 7 × 9 = 63 plus the 3 you carried is 66. Write the 66 in front of the 50 and you've got 6,650, which is the 350 and 6300 combined. Add the two lines, and you're done.
As you can see, a long multiplication problem can be broken down into a series of simple multiplication problems. It's important to have basic multiplication facts in memory, so you don't have to spend time doing the repeated addition every time. You'll learn more about that in the next chapter.
Problem 1:
Multiply 73 by 5.
Solution:
Begin with 5 × 3 = 15. Put down the 5 and carry 1. Then 5 × 7 is 35 plus the 1 you carried is 36.
Problem 2:
Find the product of 86 and 12.
Solution:
First multiply 86 by 2.
Multiplying 2 × 6 gives you 12, so put down the 2 and carry 1. Then 2 × 8 is 16 plus 1 you carried is 17.
Place a zero at the end of the second line, or if you prefer, just move one space right, and multiply 1 × 86, which obviously is 86. Add the columns to complete the job.
Problem 3:
What is the result when 125 is multiplied by 32?
Solution:
Multiplying 125 by 2 requires a little bit of carrying.
Place a zero on the second line, or move one space right, then multiply 3 × 125. Add down each column, regrouping where necessary.
Ready to test yourself? Try Self-Test 2.1.
SELF-TEST 2.1
Add 453 and 975.
Find the sum of 1,864 and 798.
Multiply 561 by 92.
What is the product of 891 and 30?
Multiply 135 × 112.
If the first two gave you trouble, review Frame 1. If you got any of the last three wrong, review Frame 2. If you've got this, move on!
3 SUBTRACTION
Subtraction is the inverse, or opposite, of addition. Addition puts together. Subtraction takes apart. If you buy a carton of 12 eggs and you use 4 of them to make breakfast, how many eggs are left? 12 – 4 = 8 if you count the remaining eggs. That subtraction problem is another way of thinking about the addition problem 4 + 8 = 12. Each subtraction problem has a related addition problem. 17 – 9 = ? is related to 9 + what = 17? Sometimes it's easier to think about the addition version. If you have 9 and you want 17, you need 1 to make 10 and then 7 more to get up to 17. That's a total of 8.
For larger problems, you'll work column by column, starting from the ones, just like addition, but sometimes you'll need to “borrow,” which means you'll regroup but in the other direction.
In the following example, the ones column is easy: 8 – 3 = 5. But trying to take 9 away from 3 in the tens column is a problem. If you only have 3, how can you take away 9? (There's more than one answer to that question. We'll look at one now and another one in Chapter 6, “Positive and Negative Numbers.”)
We only have 3 tens in our tens column, and we need more, so we're going to borrow 1 hundred from the hundreds column and exchange it for 10 tens. There are 6 hundreds, so if we borrow 1, there will be 5 left. We'll exchange the 1 hundred for 10 tens and add them to our 3 tens so we have 13 tens. We can subtract 9 tens from 13 tens and get 4 tens, then subtract 1 hundred from the remaining 5 hundreds to get 4 hundreds.
Problem 1:
Subtract 124 from 896.
Solution:
This subtraction doesn't require any regrouping. Just subtract the digits in each column: 6 – 4 = 2, 9 – 2 = 7, and 8 – 1 = 7.
Problem 2:
Find the difference between 293 and 581.
Solution:
To begin, you need to “borrow” and regroup, because you can't subtract 3 from 1. Remember that the 8 tens become 7 tens and the ten we borrowed is added to the 1. Subtract 3 from 11 to get 8.
But this requires another regrouping. Borrow 1 from the 5 hundreds, leaving 4 hundreds, and change the borrowed hundred to 10 tens. Add that to the 7 (not 8) tens and subtract 9 from 17.
Finish by subtracting 2 hundred from the remaining 4 hundred.
4 DIVISION
Division is the opposite, or inverse, of multiplication. You can think of a division question like 21 ÷ 3 = ? as the related multiplication question 3 × what = 21? Sometimes memory is enough to answer that. You can also phrase the question as, “If you put 21 objects into groups of 3, how many groups can you make?” Another approach is to ask, “If 3 people share $21, how much does each person get?”
Division with larger numbers can get complicated. There are several methods, one of which is covered below. Others will come up in Chapter 4, “Focus on Division.” It's fair to say that division with larger numbers is one task for which calculators are helpful. If you're going to use a calculator, you'll want to be able to estimate the answer, so that you can see if the answer on the calculator is reasonable. Errors, even as simple as a typing error, can give you a wildly wrong answer.
Estimation
When you use a calculator for division, it helps to have an idea of what a reasonable answer will look like. If you divide 3,482,603 by 274 and get an answer of 12, bells should go off to say that can't possibly be right. Why doesn't that sound right? Well, think about dividing with easier numbers. 3,482,603 is larger than 3,000,000 and 274 is around 300. 3,000,000 ÷ 300 = 10,000 so the answer to the actual problem should be in that neighborhood, which means 12 must be wrong. In fact, 3,482,603 ÷ 274 = 12,710 with a little left over.
Short Division
In a division problem, you have a dividend, a divisor, a quotient, and a remainder. You don't need those labels all that often but we'll need them to talk about how to divide. If you want to divide 785 by 5, the 785 is the dividend and 5 is the divisor. If the dividend is large but the divisor is small, you can use what's called short division. It's a method that uses regrouping and compresses the work.
Set up with the dividend inside and the divisor outside, like this: . Look at the first digit in the dividend, 7. Can you divide 7 by 5? There is one 5 and 2 left over. Show that like this:
Those 2 hundreds that were left get regrouped as 20 tens so now that middle number is not just 8 but 28. Divide 28 by 5, and you get 5 with 3 left over.
Divide 35 by 5 to get 7.
Problem 1:
What is the result of dividing 161 by 7?
Solution:
Divide 7 into 16. It goes twice, so place the 2 up top, but 7 × 2 = 14, so there will be 2 left over. Write the 2 beside the 1 and divide 7 into 21. That gives you 3.
Problem 2:
Divide 1,829 by 31. Estimate first by thinking about 1,800 ÷ 30.
Solution:
Estimate your answer by thinking about how many times 30 could be subtracted from 1,800 or by thinking that 18 ÷ 3 = 6 and so 1,800 ÷ 30 would need a 6 and another zero. 1,800 ÷ 30 = 60, so expect a quotient close to 60. This one may be hard to keep all in your head, so write down as you go.
You're probably tempted to start with a 6, but if you check 6 × 31 = 186, and you're trying to divide into 182, so 6 is too big. Drop down to 5, and place 5 × 31 = 155 under the 182.
Subtract 182 – 155 = 27 and bring down the 9. Three goes into 27 nine times so there's a good chance 31 goes into 279 nine times. Multiply to check it.
3Focus on Multiplication
In this chapter, we'll make reference to Chapter 2, “Essential Arithmetic,” pay a bit of attention to multiplication problems that demand greater time and attention, look at a few real-life uses of your multiplication skills, and consider when it's appropriate to use a calculator. We'll be doing some longer, more complex multiplication problems later in this chapter, but first, spend a little time checking your knowledge of multiplication facts. Having these in memory will make multiplication and estimation quicker and easier.
1 THE MULTIPLICATION TABLE
Table 3.1 goes from 1 × 1 to 10 × 10. Many tables go up to 12 × 12. But how many times do you need to recall 12 × 11?
How well do you know the table? There's only one way to find out. Fill it in. Multiply each number in the vertical column by each number in the horizontal row. A few are done to get you started.
Once you've finished, use Table 3.2 to check your work. Circle any wrong answers. The problems most commonly missed are 9 × 6 and 8 × 7.
Use Table 3.2 to check your answers.
Table 3.1 Blank Multiplication Table
Please fill in completely. If you're not too sure of yourself, do it in pencil first.
1
2
3
4
5
6
7
8
9
10
1
1
2
2
2
4
3
9
12
4
5
6
7
8
9
10
Table 3.2 Completed Multiplication Table
1
2
3
4
5
6
7
8
9
10
1
1
2
3
4
5
6
7
8
9
10
2
2
4
6
8
10
12
14
16
18
20
3
3
6
9
12
15
18
21
24
27
30
4
4
8
12
16
20
24
28
32
36
40
5
5
10
15
20
25
30
35
40
45
50
6
6
12
18
24
30
36
42
48
54
60
7
7
14
21
28
35
42
49
56
63
70
8
8
16
24
32
40
48
56
64
72
80
9
9
18
27
36
45
54
63
72
81
90
10
10
20
30
40
50
60
70
80
90
100
If you got everything right, you can skip over to Frame 3. If you got just one or two of these wrong, then you'll need to go over them several times, until you're sure you know them.
What if you just don't know the multiplication table at all? Keep reading.
2 LEARNING THE MULTIPLICATION TABLE
Let's return to a concept discussed earlier: multiplication as addition. You can complete the number sequences below by adding the same number again and again. You might have heard it called skip counting. Fill in the missing numbers of these five series:
1
2
3
__
__
__
__
__
__
__
2
4
6
__
__
__
__
__
__
__
3
6
9
__
__
__
__
__
__
__
4
8
12
__
__
__
__
__
__
__
5
10
15
__
__
__
__
__
__
__
Now check your work:
1
2
3
4
5
6
7
8
9
10
2
4
6
8
10
12
14
16
18
20
3
6
9
12
15
18
21
24
27
30
4
8
12
16
20
24
28
32
36
40
5
10
15
20
25
30
35
40
45
50
If you've got those, you're halfway through the multiplication table. Try the sixes. Even if you need to count on your fingers, that's okay. People who count on their fingers are the original digital computers.
6
12
18
__
__
__
__
__
__
__
Sevens are next:
7
14
21
__
__
__
__
__
__
__
Now check your work:
6
12
18
24
30
36
42
48
54
60
7
14
21
18
35
42
49
56
63
70
How did you do? If you got them all right, go on to the next set. If not, continue working on those you missed.
You're ready to do eights:
8
16
24
__
__
__
__
__
__
__
Now try nines:
9
18
27
__
__
__
__
__
__
__
Finally, the easy one. Fill in the missing numbers for tens:
10
20
30
__
__
__
__
