2.6 (Present value of an ordinary annuity) What is the present value of the following annuities? (a) $2500 a year for 10 years discounted to the present at 7% (b) $70 a year for three years discounted to the present at 3\% (c) \$280 a year for seven years discounted to the present at 6% (d) $500 a year for 10 years discounted to the present at 10%