[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.