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Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page iSECRE SOFMEN AL MA H

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Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page iiiSECRE SOFMEN AL MA HThe Mathemagician’s Guide to Lightning Calculationand Amazing Math TricksArthur Benjaminand Michael Shermer

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page ivCopyright 2006 by Arthur Benjamin and Michael ShermerAll rights reserved.Published in the United States by Three Rivers Press, an imprint of the CrownPublishing Group, a division of Random House, Inc., New York.www.crownpublishing.comOriginally published in different form as Mathemagics by Lowell House,Los Angeles, in 1993.Three Rivers Press and the Tugboat design are registered trademarks of RandomHouse, Inc.Library of Congress Cataloging-in-Publication DataBenjamin, Arthur.Secrets of mental math : the mathemagician’s guide to lightning calculation andamazing math tricks / Arthur Benjamin and Michael Shermer.— 1st ed.p. cm.Includes bibliographical references and index.1. Mental arithmetic—Study and teaching. 2. Magic tricks in mathematicseducation. 3. Mental calculators. I. Shermer, Michael. II. Title.QA111.B442006510—dc222005037289eISBN: 978-0-307-34746-6v1.0

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page vI dedicate this book to my wife, Deena,and daughters, Laurel and Ariel.—Arthur BenjaminMy dedication is to my wife, Kim,for being my most trusted confidanteand personal counselor.—Michael Shermer

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Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page viiAcknowledgmentsThe authors wish to thank Steve Ross and Katie McHugh atRandom House for their support of this book. Special thanks toNatalya St. Clair for typesetting the initial draft, which waspartly supported by a grant from the Mellon Foundation.Arthur Benjamin especially wants to acknowledge those whoinspired him to become both a mathematician and a magician—cognitive psychologist William G. Chase, magicians Paul Gertnerand James Randi, and mathematicians Alan J. Goldman andEdward R. Scheinerman. Finally, thanks to all of my colleaguesand students at Harvey Mudd College, and to my wife, Deena,and daughters, Laurel and Ariel, for constant inspiration.

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Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page ixContentsForeword by Bill Nye (the Science Guy )xiForeword by James RandixviiPrologue by Michael ShermerxixIntroduction by Arthur BenjaminxxiiiChapter 0 Quick Tricks:Easy (and Impressive) Calculations1Chapter 1 A Little Give and Take:Mental Addition and Subtraction11Chapter 2 Products of a Misspent Youth:Basic Multiplication29Chapter 3 New and Improved Products:Intermediate Multiplication53Chapter 4 Divide and Conquer:Mental Division80Chapter 5 Good Enough:The Art of “Guesstimation”108

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xxContentsChapter 6 Math for the Board:Pencil-and-Paper Math131Chapter 7 A Memorable Chapter:Memorizing Numbers151Chapter 8 The Tough Stuff Made Easy:Advanced Multiplication163Chapter 9 Presto-digit-ation:The Art of Mathematical Magic199Chapter Epilogue by Michael Shermer:How Math Helps Us Think AboutWeird Things222Answers233Bibliography271Index273

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xiForewordby Bill Nye (the Science Guy )I like to think about the first humans, the people who came upwith the idea to count things. They must have noticed rightaway that figuring on your fingertips works great. Perhaps Og(a typical ancient cave guy) or one of his pals or associates said,“There are one, two, three, four, five of us here, so we need fivepieces of fruit.” Later, “Hey, look,” someone must have said (orgrunted), “you can count the number of people at the campfire,the number of birds on a tree, stones in a row, logs for a fire, orgrapes in a bunch, just with your fingers.” It was a great start.It’s probably also how you came to first know numbers.You’ve probably heard that math is the language of science,or the language of Nature is mathematics. Well, it’s true. Themore we understand the universe, the more we discover itsmathematical connections. Flowers have spirals that line upwith a special sequence of numbers (called Fibonacci numbers)that you can understand and generate yourself. Seashells formin perfect mathematical curves (logarithmic spirals) that comefrom a chemical balance. Star clusters tug on one another in amathematical dance that we can observe and understand frommillions and even billions of kilometers away.We have spent centuries discovering the mathematical natureof Nature. With each discovery, someone had to go through the

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xiixiiForewordmath and make sure the numbers were right. Well, Secrets ofMental Math can help you handle all kinds of numbers. You’llget comfortable with calculations in a way that will let you knowsome of Nature’s numerical secrets, and who knows where thatmight take you?As you get to know numbers, the answer really is at yourfingertips. That’s not a joke, because that’s where it all begins.Almost everyone has ten fingers, so our system of mathematicsstarted with 1 and went to 10. In fact, we call both our numbers and our fingers “digits.” Coincidence? Hardly. Prettysoon, though, our ancestors ran out of fingers. The same thinghas probably happened to you. But we can’t just ignore thosebig numbers and (this is a joke) throw up our hands.We need numbers—they’re part of our lives every day, and inways we typically don’t even notice. Think about a conversationyou had with a friend. To call, you needed a phone number, andthe time you spent on the phone was measured in numbers ofhours and minutes. Every date in history, including an important one like your birthday, is reckoned with numbers. We evenuse numbers to represent ideas that have nothing to do withcounting. What’s your 20? (I.e., Where are you? From the oldpolice “10” codes, like 10-4 for “yes.”) What’s the 411 on thatgal? (I.e., What’s her background; is she dating anyone? Fromthe number for telephone information.) People describe oneanother in numbers representing height and weight. And, ofcourse, we all like to know how much money we have or howmuch something costs in numbers: dollars, pesos, yuan, rupees,krona, euros, or yen. Additionally (another joke), this book hasa time-saving section on remembering numbers—and large numbers of numbers.If, for some reason, you’re not crazy about math, read a littlefurther. Of course I, as the Science Guy, hope you do like math.

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xiiiForewordWell, actually, I hope you love math. But no matter how you feelabout math, hatred or love, I’d bet that you often find yourselfjust wanting to know the answer right away, without havingto write down everything carefully and work slowly and diligently—or without even having to stop and grab a calculator.You want the answer, as we say, “as if by magic.” It turns outthat you can solve or work many, many math problems almostmagically. This book will show you how.What makes any kind of magic so intriguing and fun is thatthe audience seldom knows how the trick is performed. “Howdid she do that . . . ?” “I don’t know, but it’s cool.” If you havean audience, the tricks and shortcuts in Secrets of Mental Mathare a lot like magic. The audience seldom knows how a trick isperformed; they just appreciate it. Notice, though, that inmagic, it’s hardly worth doing if no one is watching. But withSecrets, knowing how it works doesn’t subtract from the fun (orpun). When arithmetic is easy, you don’t get bogged down in thecalculating; you can concentrate on the wonderful nature ofnumbers. After all, math runs the universe.Dr. Benjamin got into this business of lightning-fast calculating just for fun. We have to figure he impressed his teachers andclassmates. Magicians might make some in their audience thinkthat they have supernatural powers. Mathemagicians, at first,give the impression that they’re geniuses. Getting people tonotice what you’re doing is an old part of sharing ideas. Ifthey’re impressed, they’ll probably listen to what you have tosay. So try some “mathemagics.” You may impress your friends,all right. But you’ll also find yourself performing just for yourself. You’ll find you’re able to do problems that you didn’t thinkyou could. You’ll be impressed . . . with yourself.Now, counting on your fingers is one thing (one finger’sworth). But have you ever found yourself counting out loud orxiii

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xivxiv Forewordwhispering or making other sounds while you calculate? Italmost always makes math easier. The problem, though, is thatother people think you’re a little odd . . . not even (more mathhumor). Well, in Secrets of Mental Math, Dr. Benjamin helpsyou learn to use that “out-loud” feature of the way your brainworks to do math problems more easily, faster, and more accurately (which is surprising), all while your brain is thinkingaway—almost as if you’re thinking out loud.You’ll learn to move through math problems the same waywe read in English, left to right. You’ll learn to handle big problems fast with good guesses, actually great guesses, within a percent or so. You will learn to do arithmetic fast; that way you canspend your time thinking about what the numbers mean. Ogwondered, “Do we have enough fruit for each person sittingaround the fire? If not, there might be trouble.” Now you mightwonder, “Is there enough space on this computer to keep trackof my music files . . . or my bank account? If not, there might betrouble.”There’s more to Secrets than just figuring. You can learn totake a day, month, and year, then compute what day of the weekit was or will be. It’s fantastic, almost magical, to be able to tellsomeone what day of the week she or he was born. But, it’sreally something to be able to figure that the United States hadits first big Fourth of July on a Thursday in 1776. April 15,1912, the day the Titanic sank, was a Monday. The first humanto walk on the moon set foot there on July 20, 1969, a Sunday.You’ll probably never forget that the United States was attackedby terrorists on September 11, 2001. With Secrets of MentalMath, you’ll always be able to show it was a Tuesday.There are relationships in Nature that numbers describe better than any other way we know. There are simple numbers thatyou can count on your hands: one, two, three, and on up. But

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xvForewordthere are also an infinite number of numbers in between. Thereare fractions. There are numbers that never end. They get as bigas you want and so small that they’re hard to imagine. You canknow them. With Secrets of Mental Math, you can have eventhese in-between numbers come so quickly to your mind thatyou’ll have a bit more space in your brain to think about whyour world works this way. One way or another, this book willhelp you see that in Nature, it all adds up.xv

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Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xviiForewordby James RandiMathematics is a wonderful, elegant, and exceedingly useful language. It has its own vocabulary and syntax, its own verbs, nouns,and modifiers, and its own dialects and patois. It is used brilliantlyby some, poorly by others. Some of us fear to pursue its more esoteric uses, while a few of us wield it like a sword to attack andconquer income tax forms or masses of data that resist the lesscourageous. This book does not guarantee to turn you into a Leibniz, or put you on stage as a Professor Algebra, but it will, I hope,bring you a new, exciting, and even entertaining view of what canbe done with that wonderful invention—numbers.We all think we know enough about arithmetic to get by, andwe certainly feel no guilt about resorting to the handy pocketcalculator that has become so much a part of our lives. But, justas photography may blind us to the beauty of a Vermeer painting, or an electronic keyboard may make us forget the magnificence of a Horowitz sonata, too much reliance on technologycan deny us the pleasures that you will find in these pages.I remember the delight I experienced as a child when I wasshown that I could multiply by 25 merely by adding two 0s to mynumber and dividing by 4. Casting out 9s to check multiplicationcame next, and when I found out about cross-multiplying I washooked and became, for a short while, a generally unbearable

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xviiixviii Forewordmath nut. Immunizations against such afflictions are not available. You have to recover all by yourself. Beware!This is a fun book. You wouldn’t have it in your hands rightnow if you didn’t have some interest either in improving yourmath skills or in satisfying a curiosity about this fascinating subject. As with all such instruction books, you may retain and useonly a certain percentage of the varied tricks and methodsdescribed here, but that alone will make it worth the investmentof your time.I know both the authors rather well. Art Benjamin is not onlyone of those whiz kids we used to groan about in school butalso has been known to tread the boards at the Magic Castle inHollywood, performing demonstrations of his skill, and on oneoccasion he traveled to Tokyo, Japan, to pit his math skillsagainst a lady savant on live television. Michael Shermer, withhis specialized knowledge of science, has an excellent overviewof practical applications of math as it is used in the real world.If this is your first exposure to this kind of good math stuff, Ienvy you. You’ll discover, as you come upon each delicious newway to attack numbers, that you missed something in school.Mathematics, particularly arithmetic, is a powerful and dependable tool for day-to-day use that enables us to handle our complicated lives with more assurance and accuracy. Let Art andMichael show you how to round a few of the corners and cutthrough some of the traffic. Remember these words of Dr. SamuelJohnson, an eminently practical soul in all respects: “Arithemetical inquiries give entertainment in solitude by the practice, andreputation in public by the effect.”Above all, enjoy the book. Let it entertain you, and have funwith it. That, with the occasional good deed, a slice of pizza (noanchovies!), and a selection of good friends is about all you canask of life. Well, almost all. Maybe a Ferrari . . .

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xixPrologueby Michael ShermerMy good friend Dr. Arthur Benjamin, mathematics professor atHarvey Mudd College in Claremont, California, takes the stageto a round of applause at the Magic Castle, a celebrated magicclub in Hollywood, where he is about to perform “mathemagics,” or what he calls the art of rapid mental calculation. Artappears nothing like a mathematics professor from a prestigiouscollege. Astonishingly quick-witted, he looks at home with therest of the young magicians playing at the Castle—which he is.What makes Art so special is that he can play in front of anygroup, including professional mathematicians and magicians,because he can do something that almost no one else can. ArtBenjamin can add, subtract, multiply, and divide numbers in hishead faster than most people can with a calculator. He cansquare two-digit, three-digit, and four-digit numbers, as well asfind square roots and cube roots, without writing anythingdown on paper. And he can teach you how to perform your ownmathematical magic.Traditionally, magicians refuse to disclose how they performtheir tricks. If they did, everyone would know how they aredone and the mystery and fascination of magic would be lost.But Art wants to get people excited about math. And he knowsthat one of the best ways to do so is to let you and other readers

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xxxxProloguein on his secrets of “math genius.” With these skills, almost anyone can do what Art Benjamin does every time he gets on stageto perform his magic.This particular night at the Magic Castle, Art begins by asking if anyone in the audience has a calculator. A group of engineers raise their hands and join Art on the stage. Offering to testtheir calculators to make sure they work, Art asks a memberof the audience to call out a two-digit number. “Fifty-seven,”shouts one. Art points to another who yells out, “Twenty-three.”Directing his attention to those on stage, Art tells them:“Multiply 57 by 23 on the calculator and make sure you get1311 or the calculators are not working correctly.” Art waitspatiently while the volunteers finish inputting the numbers. Aseach participant indicates his calculator reads 1311, the audience lets out a collective gasp. The amazing Art has beaten thecalculators at their own game!Art next informs the audience that he will square four twodigit numbers faster than his button-pushers on stage cansquare them on their calculators. The audience asks him tosquare the numbers 24, 38, 67, and 97. Then, in large, boldwriting for everyone to see, Art writes: 576, 1444, 4489, 9409.Art turns to his engineer volunteers, each of whom is computinga two-digit square, and asks them to call out their answers.Their response triggers gasps and then applause from the audience: “576, 1444, 4489, 9409.” The woman next to me sitswith her mouth open in amazement.Art then offers to square three-digit numbers without evenwriting down the answer. “Five hundred and seventy-two,” agentleman calls out. Art’s reply comes less than a second later:“572 squared is 327,184.” He immediately points to anothermember of the audience, who yells, “389,” followed by Art’sunblinking response: “389 squared will give you 151,321.”

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xxiPrologueSomeone else blurts out, “262.” “That’ll give you 68,644.” Sensing he delayed just an instant on that last one, he promises tomake up for it on the next number. The challenge comes—991.With no pause, Art squares the number, “982,081.” Severalmore three-digit numbers are given and Art responds perfectly.Members of the audience shake their heads in disbelief.With the audience in the palm of his hand, Art now declaresthat he will attempt to square a four-digit number. A woman callsout, “1,036,” and Art instantly responds, “That’s 1,073,296.”The audience laughs and Art explains, “No, no, that’s much tooeasy a number. I’m not supposed to beat the calculators on these.Let’s try another one.” A man offers a challenging 2,843. Pausingbriefly between digits, Art responds: “Let’s see, the square of thatshould be 8 million . . . 82 thousand . . . 649.” He is right, ofcourse, and the audience roars their approval, as loudly as theydid for the previous magician who sawed a woman in half andmade a dog disappear.It is the same everywhere Art Benjamin goes, whether it is ahigh school auditorium, a college classroom, a professional conference, the Magic Castle, or a television studio. Professor Benjamin has performed his special brand of magic live all over thecountry and on numerous television talk shows. He has been thesubject of investigation by a cognitive psychologist at CarnegieMellon University and is featured in a scholarly book by StevenSmith called The Great Mental Calculators: The Psychology,Methods, and Lives of Calculating Prodigies, Past and Present.Art was born in Cleveland on March 19, 1961 (which he calculates was a Sunday, a skill he will teach you in Chapter 9). Ahyperactive child, Art drove his teachers mad with his classroom antics, which included correcting the mathematical mistakes they occasionally made. Throughout this book whenteaching you his mathematical secrets, Art recalls when andxxi

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xxiixxiiProloguewhere he learned these skills, so I will save the fascinating stories for him to tell you.Art Benjamin is an extraordinary individual with an extraordinary program to teach you rapid mental calculation. I offerthese claims without hesitation, and ask only that you remember this does not come from a couple of guys promising miraclesif you will only call our 800 hotline. Art and I are both credentialed in the most conservative of academic professions—Art inmathematics and I, myself, in the history of science—and wewould never risk career embarrassment (or worse) by makingsuch powerful claims if they were not true. To put it simply, thisstuff works, and virtually everyone can do it because this art of“math genius” is a learned skill. So you can look forward toimproving your math skills, impressing your friends, enhancingyour memory, and, most of all, having fun!

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xxiiiIntroductionEver since I was a child, I have loved playing with numbers,and in this book I hope to share my passion with you. I havealways found numbers to have a certain magical appeal andspent countless hours entertaining myself and others with theirbeautiful properties. As a teenager, I performed as a magician,and subsequently combined my loves of math and magic into afull-length show, called Mathemagics, where I would demonstrate and explain the secrets of rapid mental calculation toaudiences of all ages.Since earning my PhD, I have taught mathematics at HarveyMudd College, and I still enjoy sharing the joy of numbers withchildren and adults throughout the world. In this book, I willshare all of my secrets for doing math in your head, quickly andeasily. (I realize that magicians are not supposed to reveal theirsecrets, but mathemagicians have a different code of ethics.Mathematics should be awe inspiring, not mysterious.)What will you learn from this book? You will learn to domath in your head faster than you ever thought possible. After alittle practice, you will dramatically improve your memory fornumbers. You will learn feats of mind that will impress yourfriends, colleagues, and teachers. But you will also learn to viewmath as an activity that can actually be fun.

Benj 0307338401 4p fm r1.r.qxd 5/4/06 1:37 PM Page xxivxxiv IntroductionToo often, math is taught as a set of rigid rules, leaving littleroom for creative thinking. But as you will learn from Secrets,there are often several ways to solve the same problem. Largeproblems can be broken down into smaller, more manageablecomponents. We look for special features to make our problemseasier to solve. These strike me as being valuable life lessons thatwe can use in approaching all kinds of problems, mathematicaland otherwise.“But isn’t math talent something that you are born with?” Iget this question all the time. Many people are convinced thatlightning calculators are prodigiously gifted. Maybe I was bornwith some curiosity about how things work, whether it be amath problem or a magic trick. But I am convinced, based onmany years of teaching experience, that rapid math is a skill thatanyone can learn. And like any worthwhile skill, it takes practice and dedication if you wish to become an expert. But toachieve these results efficiently, it is important that you practicethe right way. Let me show you the way!Mathemagically,Dr. Arthur BenjaminClaremont, California

Benj 0307338401 4p c00 r1.r.qxd 5/4/06 1:46 PM Page 1Chapter 0Quick Tricks:Easy (and Impressive) CalculationsIn the pages that follow, you will learn to do math in your headfaster than you ever thought possible. After practicing the methods in this book for just a little while, your ability to work withnumbers will increase dramatically. With even more practice,you will be able to perform many calculations faster than someone using a calculator. But in this chapter, my goal is to teachyou some easy yet impressive calculations you can learn to doimmediately. We’ll save some of the more serious stuff for later.INSTANT MULTIPLICATIONLet’s begin with one of my favorite feats of mental math—howto multiply, in your head, any two-digit number by eleven. It’svery easy once you know the secret. Consider the problem:32 11, putTo solve this problem, simply add the digits, 3 2 5 the 5 between the 3 and the 2, and there is your answer:

Benj 0307338401 4p c00 r1.r.qxd 5/4/06 1:46 PM Page 22 Secrets of Mental Math35 2What could be easier? Now you try:53 11Since 5 3 8, your answer is simply583One more. Without looking at the answer or writing anything down, what is81 11?Did you get 891? Congratulations!Now before you get too excited, I have shown you only halfof what you need to know. Suppose the problem is85 11Although 8 5 13, the answer is NOT 8135! 1 needsAs before, the 3 goes in between the numbers, but the to be added to the 8 to get the correct answer:935 Think of the problem this way:1835 935

Benj 0307338401 4p c00 r1.r.qxd 5/4/06 1:46 PM Page 3Quick Tricks: Easy (and Impressive) CalculationsHere is another example. Try 57 11.Since 5 7 12, the answer is1527 627Okay, now it’s your turn. As fast as you can, what is77 11?If you got the answer 847, then give yourself a pat on theback. You are on your way to becoming a mathemagician.Now, I know from experience that if you tell a friend orteacher that you can multiply, in your head, any two-digit number by eleven, it won’t be long before they ask you to do 99 11. Let’s do that one now, so we are ready for it.Since 9 9 18, the answer is:1989 1089Okay, take a moment to practice your new skill a few times,then start showing off. You will be amazed at the reaction youget. (Whether or not you decide to reveal the secret is up to you!)Welcome back. At this point, you probably have a few questions, such as:“Can we use this method for multiplying three-digit numbers(or larger) by eleven?”3

Benj 0307338401 4p c00 r1.r.qxd 5/4/06 1:46 PM Page 44 Secrets of Mental MathAbsolutely. For instance, for the problem 314 11, theanswer still begins with 3 and ends with 4. Since 3 1 4 , and1 4 5 , the answer is 34 5 4. But we’ll save larger problemslike this for later.More practically, you are probably saying to yourself,“Well, this is fine for multiplying by elevens, but what aboutlarger numbers? How do I multiply numbers by twelve, orthirteen, or thirty-six?”My answer to that is, Patience! That’s what the rest of thebook is all about. In Chapters 2, 3, 6, and 8, you will learn methods for multiplying together just about any two numbers. Betterstill, you don’t have to memorize special rules for every number.Just a handful of techniques is all that it takes to multiply numbers in your head, quickly and easily.SQUARING AND MOREHere is another quick trick.As you probably know, the square of a number is a numbermultiplied by itself. For example, the square of 7 is 7 7 49.Later, I will teach you a simple method that will enable you toeasily calculate the square of any two-digit or three-digit (orhigher) number. That method is especially simple when thenumber ends in 5, so let’s do that trick now.To square a two-digit number that ends in 5, you need toremember only two things.1. The answer begins by multiplying the first digit by the next higherdigit.2. The answer ends in 25.

Benj 0307338401 4p c00 r1.r.qxd 5/4/06 1:46 PM Page 5Quick Tricks: Easy (and Impressive) CalculationsFor example, to square the number 35, we simply multiplythe first digit (3) by the next higher digit (4), then attach 25.Since 3 4 12, the answer is 1225. Therefore, 35 35 1225. Our steps can be illustrated this way:35 35 3 4 12255 5 Answer: 1225How about the square of 85? Since 8 9 72, we immediately get 85 85 7225.85 85 8 9 72255 5 Answer: 7225We can use a similar trick when multiplying two-digit numbers with the same first digit, and second digits that sum to 10.The answer begins the same way that it did before (the firstdigit multiplied by the next higher digit), followed by the product of the second digits. For example, let’s try 83 87. (Bothnumbers begin with 8, and the last digits sum to 3 7 10.)Since 8 9 72, and 3 7 21, the answer is 7221.83 87 8 9 72213 7 Answer: 72215

Benj 0307338401 4p c00 r1.r.qxd 5/4/06 1:46 PM Page 66 Secrets of Mental MathSimilarly, 84 86 7224.Now it’s your turn. Try26 24How does the answer begin? With 2 3 6. How does itend? With 6 4 24. Thus 26 24 624.Remember that to use this method, the first digits have to bethe same, and the last digits must sum to 10. Thus, we can usethis method to instantly determine that31 39 120932 38 121633 37 122134 36 122435 35 1225You may ask,“What if the last digits do not sum to ten? Can we use thismethod to multiply twenty-two and twenty-three?”Well, not yet. But in Chapter 8, I will show you an easy way todo problems like this using the close-together method. (For 22 23, you would do 20 25 plus 2 3, to get 500 6 506, butI’m getting ahead of myself!) Not only will you learn how to usethese methods, but you will understand why these methodswork, too.“Are there any tricks for doing mental addition andsubtraction?”Definitely, and that is what the next chapter is all about. If Iwere forced to summarize my method in three words, I wouldsay, “Left to right.” Here is a sneak preview.

Benj 0307338401 4p c00 r1.r.qxd 5/4/06 1:46 PM Page 7Quick Tricks: Easy (and Impressive) CalculationsConsider the subtraction problem1241 587 Most people would not like to do this problem in their head(or even on paper!), but let’s simplify it. Instead of subtracting587, subtract 600. Since 1200 600 600, we have that1241 600 641But we have subtracted 13 too much. (We will explain how toquickly determine the 13 in Chapter 1.) Thus, our painfull

You’ve probably heard that math is the language of science, or the language of Nature is mathematics. Well, it’s true. The more we understand the universe, the more we discover its mathematical connections. Flowers have spirals that line up with a special sequence of numbers (called Fi