[wlesieutre]

Math nitpick: It's a bit more complicated than that with exponential growth. You can't divide up 100% over 10 years and say it was 10% each year, instead you have to solve 2=1.1^x, and find that it doubles in 7.27 years.

If something grows 10% a year for 10 years, it actually ends up at 2.59x its original size.

[greglindahl]

Rule of 72: doubles in 10 years, annual %age is 72/10 = 7.2%. Close to the right answer, easy to do in your head.

http://en.wikipedia.org/wiki/Rule_of_72

[wlesieutre]

Huh, that's a handy approximation. I'd never heard of it before.

[smoyer]

You might not have heard of it, but every financial planner in the world can do this math in their head!

[pacaro]

Also: pi-seconds is a nano-century.

[dTal]

My favorite: 1337% of pi = 42 Beats Euler's identity any day.