When you purchased your house, you took out a $30-$yar annual$-$payment mortgage with an interest rate of $6\%$ per year. The annual payment on the mortgage is S$1200.$ You have just made a payment and have now decided to pay the mortgage off by repaying the outstanding balance. What is the payoff amount ifa. You have lived in the house for $12$ years $($so there are $18$ years left on the mortgage$)?$b$.$ You have lived in the house for $20$ years $($so there are $10$ years left on the mortgage$)?$c$.$ You have lived in the house for $12$ years $($so there are $18$ years left on the mortgage$)$ and you decide to pay off the mortgage immediately before the twelfth payment is due?