Compute the present value of a $250 cash flow for the following combinations of discount rates and times. (Do not round intermediate calculations. Round your answers to 2 decimal places.) a. r= 8%; t = 10 years b. r= 8%; t= 20 years c. r=4%; t = 10 years d. r=4%; t = 20 years a. Present value b. Present value C. Present value d. Present value