Question

Albert purchased a house for $1,500,000 fifteen years ago. He put 30% down and financed the balance by a 20-year real estate mortgage at 6%, convertible monthly. Albert decides to pay the remaining loan balance in full by a single payment together with the installment just due. Find the prepayment penalty, which is one-third of the lenders interest loss.

Please explain and do not copy from Chegg. Otherwise i have to report the answer.