Paul invested $800 at the end of every month into an investment fund that was earning interest at 4.86% compounded monthly. He stopped making regular deposits at the end of 7 years when the interest rate changed to 6.63% compounded quarterly. However, he let the money grow in this investment fund for the next 3 years.

a. Calculate the accumulated balance in his investment fund at the end of 7 years.

b. Calculate the accumulated balance in his investment fund at the end of 11 years