Suppose you pick a number between 1 and 9 (more than 1 and less than 10). To keep things general (and see why the “trick” works), I’ll call this number x. Following the numbered instructions, we get…

1. x
2. 2x
3. 2x + 5
4. 50(2x + 5) = 100x + 250
5. 100x + 250 +1759 = 100x + 2009 OR 100x + 250 + 1758 = 100x + 2008
(See why the “trick” works? It forces you to 2008 or 2009, depending on your birthday.)
6. If you subtract the four-digit year of your birthday, what you get is…
100x + (2009 – birth year) OR 100x + (2008 – birth year)
…but the stuff in parentheses is just how you calculate your age (current year minus birth year). The multiplication by 100 (really by 2 and then by 50) forces the single-digit number you chose into the third position (the “hundreds place”).

There are two caveats here. First, it will only work (as written) if your age is a two-digit number. So, if you’re an old woman (100+) who likes chocolate, you’re out of luck. Second, the claim that this is the only year it will work is a little silly. It will only work AS WRITTEN in 2009, but it’s easy to see how we could change the rules so it works in future years. You get some parameters (the numbers we multiplied by and added) to play with. In 2010, use 1760/1759…in 2011, use 1761/1760…etc. You could also manipulate the values to accommodate older folks, but the number of centenarians reading this sort of email is likely quite small.

You’re all welcome.