What formula would I use?

Example 5-3: Suppose you can buy a security at a price of $78.35 and it will pay you $100 after five years. What interest rate would you earn if you bought this security (or What discount rate is implicit in this investment?) Example 5-4: Suppose you invest $78.35 at an interest rate of 5% per year. How long will it take your investment to grow to $100