Sally begins planning for retirement at the age of 22 by making monthly investments of $250 into an account where interest is 7.5% compounded monthly. Sally continues making her monthly investments of $250 from age 22-42, and then at 42 Sally increases her monthly investments to $600. Sally continues making the indicated monthly investments until she retires at age 62. Use this to complete each of the following.

(a) Find the balance of Sallys account when she retires at the age of 62.

(b) Determine how much Sally has earned in interest at the time of her retirement.