The figure below is a contour diagram of the monthly payment on a $5-$year car loan as a function of the interest rate and the amount borrowed for a used car.

What is the interest rate if the monthly payment is $$80$ when you borrow $$4000,$

Give your answer to the nearest percent.

$\%$

$($b$)$ Suppose you borrow $$5000$ at an interest rate of $7\%.$ How much more money could you borrow if the interest rate was only $4\%$ instead?