Jew Central- Jewish News for Jewish Professionals - Know 72: Do calculations in your head

There are times when you want to quickly evaluate how quickly your money will double at a given interest rate. Then just use the 72 rule- just divide the interest rate into 72. For example, if you want to know how long it will take to double your money at eight percent interest, divide 8 into 72 and get 9 years. If your interest rate goes up to ten percent, then it's only 7.2 years.

Want proof this simple trick works? See The Rule of 72 - Why it Works.