© 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 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