a. Your parents will retire in 15 years. They currently have $320,000 saved, and they think they will need $1,200,000 at retirement. What annual interest rate must they earn to reach their goal, assuming they don't save any additional funds? Round your answer to two decimal places. b. If you deposit money today in an account that pays 11.5% annual interest, how long will it take to double your money? Round your answer to two decimal places