What is the total interest paid on a 15 year $250,000 mortgage at 7% annual interest rate. Assume that the mortgage is kept for the full 15 years. b) What are the key advantages of investing in a 401(k)? a) Use the Rule of 72 to determine how long it will take an investment that has a return of 6% annually to quadruple in value. b) As 2a, but solve exactly (Don't use Rule of 72)