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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




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


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


Also: pi-seconds is a nano-century.


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




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