A $100,000 mortgage at 6.9% compounded semiannually with a 25-year amortization requires monthly payments. The mortgage entitles the borrower to increase the amount of the regular payment by up to 15% once each year. How much will the amortization period be shortened if, after the twelfth payment, the payments are increased by:
a. 7.5%?
b. 15%?