Do Speedy Math in Your Head

From Wired How-To Wiki

Arthur Benjamin is a wizard at math. Literally. At Hollywood's Magic Castle, the world-famous conjurer's club, he wows the crowd by multiplying big numbers — quick, what's 57,682 squared? — faster than you can use a calculator. Here he shares three cool tricks. (The answer, by the way: 3,327,213,124. See how easy it is?)

Square off

To square a number like 14, identify the closest round number — in this case, 10. Since you subtracted 4 to get 10, add 4 to 14 to get 18 and multiply that by 10. Add to that the square of 4: 180 + 16 = 196.

11 times any two-digiter

To multiply, say, 11 x 32, add the digits of 32 (3 + 2 = 5) and insert the sum between them: 352. Numbers with two-digit sums use a slight variation: For 11 x 84 (8 + 4 = 12), add the 1 from 12 to the 8 and leave the 2 in the middle: 924.

Magic number

Ask a fan to think of any number. Then have them double it, add 12, divide by 2, and subtract the original number. Before they're done, tell them the answer: 6. It will always be 6.

Contributed by Katharine Gammon

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Yeah, I'm just having a bit of fun tonight. Now if we can just get Paul and Paul to give us problems that fit these limitations, we'll all be able to follow along. ... right guys.

Tags: 11, math, square

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Comment by Allen R. Gale on November 17, 2009 at 12:19pm: I agree about the "tricks", but do regret the lack of teaching of the skills. I have had slide rules break, calculators fail (at som e very inopportune times), computers fail, but my math skills didn't fail - they are just wearing out from lack of use! If you learn the basic skills and principles of ANYthing, you will not be lost if your "helps" fail you.

Jim, that mechanical calculator you used was probably a Friden, right? I knew them well. They took the drudgery out of repeat calculations but you still had to know what you are doing. Last year, in a local high school I was subbing in, I was introduced to graphical calculators - what I wouldn't have given for one of those in 1965! You still have to know basic math and algebra to set them up, but they make a lot fewer arithmetic errors than I used to.

Comment by Jim Martin on November 17, 2009 at 11:10am: Calculators are accurate and require little training. While some basic arithmetic skills are useful, I do not get concerned about teaching people these advanced tricks. Before calculators, I used slide rules and got pretty good answers. I remember using a mechanical calculator at NASA that could give square roots in about 15 seconds. Later, we could get an electronic calculator that did trig for under $500. Now, I have been given several calculators at trade shows.

An interesting point is that people in developing countries do not need to learn how to make calculators. They can buy them and easily learn to use them. That is an example of how technical progress can be beneficial to all people.

Comment by W. Dan Chance on November 17, 2009 at 8:22am: Allen I like your solution better than the other expert.

Comment by Paul on November 17, 2009 at 6:53am: Hey Chris,

Two bank account numbers for ya with Gideon bank: 875 020 079 and 875 020 709 Please be careful which one you use, as one is for deposits the other is for withdrawals.

"Ahhh suppressed laughter is truly the best." -- George Carlin

Comment by Allen R. Gale on November 17, 2009 at 5:33am: I think I recall seeing this guy on "early" TV. Being one who has enjoyed a huge aptitude for math, I used to be able to do mental math faster than using a calculator (actually the only portable "calculator" I had in H.S. was a slide rule!) the "11" shortcut is slower and more cumbersome than simply multiplying by 10 - which is simply adding a zero at the end or moving the decimal point one place right, then adding the number since multiplication is just repetitive addition.

Comment by Lou De Frog on November 17, 2009 at 4:48am: Hi Paul.

Just heard from the Fed and it looks as if they have decided to print some money for us. I need a bank account number so I can transfer funds to you. I have already lost too many trunks of money to corrupt agents and delivery services so it will have to be a bank to bank transfer.

Please stick to the big whole numbers when you let me know what you need. I am really terrible with all those little fractions and things.

Comment by Paul on November 16, 2009 at 8:11pm: Oh that's too funny! (not) .. But I did flash back to middle school and how my math teacher used to drill the kids by playing "go to the head of the class" a la musical chairs. One kid would start at the back of the room, and stand behind the kid seated to one side. The teacher would flip a card or a small slate or just blurt out a problem. Which ever of the two got the correct answer 1st would advance the other would take the seat. Two wrong answers and the toss-up went to the room, and that person would rise and try to advance.

All pretty basic stuff, except there were a few of us... who, well, knew the tricks ^_^ The teacher was not going to allow my ability to estimate logarithms and roots to 2-3 places to dominate the class. So, invariable when it was my turn to compete for speed, he'd toss out the most *humiliating* problems... My last memorable victory was 7^7 seven to the seventh power = 823543, and from that point on I and my challengers would face laughter inducing challenges like "3+1".

Thanks for the laugh!

"Sometimes the deck is stacked, other times the deck is stacked against you." -- ctyankee