I just need an explanation with the answer on number 20. Thanks

Solve the problem. Use an annual percentage rate table if necessary. 20) Sarah Yost took out a 48-month fixed installment loan of $17,000 to open a gift shop. 20) She began making monthly payments of $423.05. Sarah's business does better than expected and instead of making her 36th payment, Sarah decides to repay her loan in full. How much interest will Sarah save (use the actuarial method)