© 2011 By: Dov Trietsch. All rights reserved

MatheMagic – Guess My Age – Method 2

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

·         Multiply your age by 2.

·         Add 7 to the result

·         Multiply the result by 5.

·         Tell me the result.

I will nonetheless immediately tell you what your age is.

How do I do this?

Let’s do the algebra.

Let A denote your age

(2A + 7) 5 = 10A + 35 so it is of the 3 digit form XY5

Now make two numbers out of the result - The last digit and the number before it. The Last digit is obviously 5, the other 2 (or 3 for a centenarian) and this number is the age + 3.

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

76 x 2 = 152

152 + 7 = 159

159 x 5 = 795

This is made of 79 and 5. And … 79 – 3 = 76

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 www.mgsltns.com/games.htm and also here: http://geekswithblogs.net/PointsToShare/archive/2011/11/15/mathemagics---guess-my-age-method-1.aspx