Have you heard of the rule of 72? Divide the annual percentage earned into 72 and you get a surprisingly accurate estimate of how many years it takes to double your money.

So the answer is about ten years.

http://c2.com/cgi/wiki?RuleOfSeventyTwo

Patrick Star wishes to determine how long it will take an initial deposit of $15,000 tp double.

a) if earns 7% annual interest on the deposit, how long it will take for him to double his money?

b) how long will it take if he can earn 9% annual interest?

c) reviewing your findings in parts a and b, indicate what relationship exists between the interest rate and the amount of time it will take Patrick Star to double his money?