The rule of 72
The rule of 72 is used to find the number of years it takes to double your money at a given interest rate.
(72/[interest rate]) = years until a principle doubles
So, how long will it take to double your money at eight percent interest? Divide 8 into 72 and you get 9 years.
If you collect 7% on your savings in a little over 10 years the amount will be double what it is today.
The rule of 72 also means that you can determine the time
it takes for a population to double if it is growing at a steady rate by solving this function:
(72/[rate of growth]) = years for population to double