To go on a summer trip, Hans borrows $$600.$ He makes no payments until the end of $2$ years, when he pays off the entire loan. The lender charges simple interest at an annual rate of $4\%.$

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

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

$ $$i$]$

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