I really like Felix's Corollary and the Millionaire's Estimation! http://en.wikipedia.org/wiki/Rule_of_72#Felix.27s_Corollary_to_the_Rule_of_72 http://en.wikip

I really like Felix's Corollary and the Millionaire's Estimation! http://en.wikipedia.org/wiki/Rule_of_72#Felix.27s_Corollary_to_the_Rule_of_72 http://en.wikipedia.org/wiki/Rule_of_72#Felix.27s_Corollary_...