11 Calculating Interest Rates. Assume the total cost of a college education will be $250,000 when your child enters college in 18 years. You presently have $25,000 to invest. What annual rate of interest must you earn on your investment to cover the cost of your child's college education? Calculating the Number of Periods. At 10 percent interest, how long does it take to double your money? To quadruple it? 12