You invest $4000 in a bank account with 5% interest compounded annually. you are wondering how long it would take for your money to double

The formula for compound interest is written as , where P is the principal (initial amount), r is the rate of interest, n is the number of times it's compounded per year, and t is the time in years.  With the values from this problem plugged in, it looks like:



Since you're trying to find when your money will double, put 8000 for A and solve for t :



It will take approximately 14.21 years, or about 14 years, 2 months, and 16 days, for the money to double.