When Would you need to use Rule 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.

The rule says that to find the number of years required to double your money at a given interest rate, you 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.