Principal amount = $800,000/1.066^(5*12) Interest in the last year = Sum Payable at the end of fifth year - Sum payable at the end of fourth year = $800,000 - $800,000*(1.066)^ (4*12 - 5*12) = $428,461.75 Solution Principal amount = $800,000/1.066^(5*12) Interest in the last year = Sum Payable at the end of fifth year - Sum payable at the end of fourth year = $800,000 - $800,000*(1.066)^ (4*12 - 5*12) = $428,461.75.