Calculate the future value of $6,000 in a. 4 years at an interest rate of 9% per year. b. 8 years at an interest rate of 9% per year. c. 4 years at an interest rate of 18% per year. d. Why is the amount of interest earned in part (a) less than half the amount of interest earned in part (b)? a. Calculate the future value of $6,000 in 4 years at an interest rate of 9% per year. The future value of $6,000 in 4 years at an interest rate of 9% per year is $ ___. (Round to the nearest dollar.)