Shruti Speak's

Rule of   "72 "      If you divide the  number 72 by the rate of interest, you get to know the  number of years it will take for you to double the money 72:   is sort of a magic number of personal finance, a.k.a known   as the rule of 72 in  personal finance parlance. All you   need to do is a quick calculation (mentally if you know   fifth grade math tables) and you can verify, if the cheeky   agent is giving you hogwash.     *Years:* If   you divide the number 72 by the rate of interest, you get to  know the number of years it will take for you to double the  > money. For instance, if the rate of interest is 9%, simply   divide the number 72 by 9% and the answer is eight. This is   the number of years it will take to double your money if you   invest it at 9% rate of interest.     *Interest:**   *This rule works in reverse order as well. Lets say, you   know the number of years you want to double your money in   and want to know the minimum rate of interest which you