Based on Exhibit $7-7,$ what would be the monthly mortgage payments for each of the following situations? $($Round your answers to $2$ decimal places.$)$

a$.$ A $$163,500,25-$year loan at $5$ percent.

b$.$ A $$221,500,20-$year loan at $5\mathrm{.}5$ percent.

c$.$ A $$203,000,25-$year loan at $5$ percent.