15. The force of interest, ?(t), is a function of time and at any time t, measured in years, is given by the formula:
?(t) = { 0.05 + 0.001t 0 ? t ? 20
{ 0.05 t > 20
a. Derive and simplify as far as possible expressions for v(t), where v(t) is the present value of a unit sum of money due at time t.
b. Calculate the present value of $100 due at the end of 25 years
c. Calculate the rate of discount per annum convertible quarterly implied by the transaction in part (b).