Loan amount = monthly repayment amount * 1-( 1+ monthly interest rate)-repayment months/monthly interest rate

8000 = 836.42 *1-(1+monthly interest rate)-15/ monthly interest rate.

Monthly interest rate =6.23543% (annual interest rate =74.82522%)

2. Calculate the one-time repayment amount

Lump-sum repayment amount = {loan amount-monthly repayment amount * 1-( 1+ monthly interest rate)-paid months/monthly interest rate} * (paid months+1).

={8000-836.42* 1-( 1+6.23543%)^-8/6.23543%}*( 1+6.23543%)^(8+ 1)

=49 19. 12 yuan

After paying off the eighth installment, you need to pay off 49 19.5438+02 yuan in one lump sum.