$[15]$ Diego wants to purchase a home with a price of $$80,000$ and has $$9,000$ for a down payment. If he can get a $25-$year mortgage at $6\%$ per year on the unpaid balance,

a$)$ What will be his equal monthly payment?

b$)$ What is the total amount he will pay before he owns the house outright?

c$)$ How much interest will he pay?