A $200,000 mortgage at 6.6% compounded semiannually with a 25-year amortization requires monthly payments. The mortgage allows the borrower to prepay up to 10% of the original principal once each year. How much will the amortization period be shortened if, on the first anniversary of the mortgage, the borrower makes (in addition to the regular payment) a prepayment of:
a. $10,000?
b. $20,000?