You borrow $500 to buy a computer. The simple interest rate is 15%. You pay off the loan after 4 years. How much do you pay for the loan?

Here the interest would be i = P*r*t, or, specifically,

interest on loan = i = ($500) (0.15) (4 yrs) = $300.

This assumes just ONE repayment: $300 plus $500, or $800, after 4 years. It's much more common to repay a loan like this with monthly payments.