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© 2011 By: Dov Trietsch. All rights reserved

MatheMagic – Guess My Age – Method 1

The Mathemagician stands on the stage and asks an adult to do the following:

·         Do the next few steps on your calculator, or the calculator in your phone, or even on a piece of paper.

·         Do it silently! Don’t tell me the results until I ask for them directly

·         Compute a single digit multiple of 9 – any one of 9, 18, 27, … all the way to 81, will do.

·         Now multiply your age by 10

·         Subtract the 9 multiple from this number.

·         Tell me the result.

Notice that I don’t know which multiple of 9 you subtracted from 10 times your age. I will nonetheless immediately tell you what your age is.

How do I do this?

Let’s do the algebra.

10X – 9Y = 10X – 10Y + Y = 10(X – Y) + Y

Now remember, you asked an adult, so his/her age is a two digit number (maybe even 3 digits), thus reducing it by the single digit multiplied by nine is still positive – the lowest is can be is 100 – 81 which yields 19.

Now make two numbers out of the result. The last digit and the number before it. This number is X – Y or the age minus the single digit you selected. The last digit is this very single digit. This is always so regardless of the digit you selected. So…

Add tis digit to the other number and you get back the age! Q.E.D

Example: I am 76 years old and here is what happens when I do the steps

76 x 10 = 760

760 – 18 = 742 made of 74 and 2. My age is 74 + 2

760 – 81 = 679 made of 67 and 9. My age is 67 + 9

A note to the socially aware mathemagician – it is safer to do it with a man. The chances of a veracious answer are much, much higher!

The trick may be accomplished on any 2 or 3 digit number, not just one’s age, but if you want to know your date’s age, it’s a good way to elicit it.

That’s All Folks

PS for more Ageless “Age” mathemagics go to and also here:



Posted on Tuesday, November 15, 2011 12:12 PM | Back to top

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