Solving for n (N). Liz can purchase a security that will provide a return of 3 percent per year. It will cost $ 1.29 to purchase, and Liz wants to keep the investment until it grows to a value of $7.50. How many years will it take the investment to grow to $7.50 (round to whole number)? 61 years 60 years 59 years 62 years 64 years