4. How much would you have to invest today to receive a. $12,000 in 6 years at 12 percent? b. $15,000 in 15 years at 8 percent? c. $5,000 each year for 10 years at 8 percent? d. $5,000 each year, at the beginning, for 10 years at 8 percent? e. $50,000 each year for 50 years at 7 percent? f. $50,000 each year for 50 years, at the beginning, at 7 percent