Mental Math Worth Its Weight

Quick, tell me what 27 x 11 is? If you answered in over 3 seconds and would like to know a trick that will render you as quick as the calculator, then read on.

For any two digit number whose sum is less than ten you can instantaneously come up with the product when multiplied by 11. Let’s look at the sample above, 27 x11. The digit in the tens column becomes the first number in your answer (2). Next, add the two digits in 27 together (2+7) to get 9. 9 is the next number in your answer. Finally, pull the 7 from the one’s column and put it in the one’s column of your answer. Hence you have 297. You will find you can do these steps very rapidly in your head and that it can help in practical situations.

Now you try one: 34 x 11. How about 43 x 11? Try doing it in your head, then check your answer with a calculator.

Secrets of Mental Math, by Arthur Benjamin and Michael Shermer is a wonderful book about math you can do quickly in your head with a few tricks. It goes far beyond the 11’s tables, and far beyond multiplication. You can check it out at most libraries and it is easy to read. I highly recommend this book!

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3 thoughts on “Mental Math Worth Its Weight

  1. That is pretty cool. It made perfect sense to me. That’s faster than my old trick of multiplying by 10 and adding the number.

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