Leila wants to buy a new house but needs money for the down payment. Her parents agree to lend her money at an annual rate of $6\%,$ charged as simple interest. They lend her $$3000$ for $4$ years. She makes no payments except the one at the end of that time.

Answer the following questions. If necessary, refer to the list of financial formulas.

$($a$)$ How much total interest will Leila have to pay?

$prod$10$

$($b$)$ What will the total repayment amount be $($including interest$)?$

$$$