George invested $1,250 at the end of every month into an investment fund that was earning interest at 4.25% compounded monthly. He stopped making regular deposits at the end of 9 years when the interest rate changed to 4.50% compounded quarterly. However, he let the money grow in this investment fund for the next 4 years.

a. Calculate the accumulated balance in his investment fund at the end of 9 years

b. Calculate the accumulated balance in his investment fund at the end of 13 years

c. Calculate the amount of interest earned over the 13-year period.