Sent me by Rick Gordon:
YOUR AGE BY CHOCOLATE MATH
• First of all, pick the number of times a week that you would like to have chocolate (more than once but less than 10).
• Multiply this number by 2 (just to be bold).
• Add 5.
• Multiply it by 50 — I’ll wait while you get the calculator.
• If you have already had your birthday this year add 1759. If you haven’t, add 1758.
• Now subtract the four digit year that you were born.
You should have a three digit number. The first digit of this was your original number (i.e., how many times you want to have chocolate each week).
The next two numbers are:
• YOUR AGE!
(This will only work in the year 2009.)
This reminded me of a carpenter’s trick someone showed me recently: take a tape measure and bring the end (tip) back around in a loop so it rests at 109″. Now look up the year you were born on the tape measure, i.e. “56” for 1956. Turn the tape over and opposite the 56 will be your age. What is going on here?
Well, it's quite simple if you write it out (I was born in 1972):
(2x + 5) * 50 + 1759 – 1972
= 100x + 250 + 1759 – 1972
= 100x + 2009 – 1972
so the 100 multiplier brings your mysterious digit into the 3rd place. 2009 is the current year and if you are under 100 the two least significant digits are your age.