15. An annuity pays 1 at the start of each year for 4n years. Using an annual effective interest rate of i, the accumulated/discounted value of the entire 4n-year annuity at time 2n is 637.72. If it is also known that (1+1)" = 8.7384, then find n. Round your answer to the nearest whole number. Hint: Break the annuity into four n-year annuities and find the value of each of these annuities at time 2n. The sum of these values will equal 637.72