mortgages are annuities in that a fiexed monthly payment is made to the lender (assume end of month payments and an interest rate that compounds semi-annually). sare is planning to take on a mortgage of $100,000 and believes she can afford monthly payments up to $700. how much interest would she save if she decided to pay off her mortgage over 20 years, rather than over 25 years? her mortgage is at five percent interest calculated semi annually.