Monthly payment = (loan principal ÷ repayment months)+(loan principal-accumulated repaid principal) × monthly interest rate. Monthly repayable principal = loan principal ÷ repayment months. Monthly interest payable = residual principal × monthly interest rate = (loan principal-accumulated principal repayment amount) × monthly interest rate. Monthly decreasing amount = monthly repayable principal × monthly interest rate = loan principal ÷ repayment months × monthly interest rate.
Total interest = [(total loan ÷ repayment months+total loan × monthly interest rate)+total loan ÷ repayment months ×( 1+ monthly interest rate) ]> 2× repayment months-total loan.