Solving for an Unknown Interest Rate 2.70 It is said that a lump-sum amount of $50,000 at the end of five years is equivalent to an equal-payment series of $5,000 per year for 10 years, where the first payment occurs at the end of year 1. What earning interest is assumed in this calculation? 2.71 At what rate of interest compounded annually will an investment double in five years? sint of cash flows shown