Assume that the force of interest is constant. The present value of a payment of 60 at the end of 10 years and 40 at the end of 20 years is equal to 40. The present value of a perpetuity with payments of P at the end of each year is also 40. What is P?

a)2.32 b)2.56 c)2.87 d)3.12 e)3.48