You would like to be a millionaire when you retire in 40 years, and how much you must invest today to reach that goal clearly depends on what rate of return you can earn. First, suppose you can earn 10.7% per year, and calculate how much you would have to invest today. Second, suppose you can only earn half that percentage rate, and calculate how much you would have to invest today. Divide the second by the first, to see how many times more you must invest today at half that annual rate grow it to $1 million over 40 years.