Generally speaking

• The customer will pay a portion of the interest that still owing but not all of it.
• Generally it works about to about half of the interest but this is by no means exact.

More specific calculations

• The interest rate that the customer pays is discounted. This exact discount between financier to financier. Usually somewhere between 2%-4%.
• Using the remaining principal owning, repayments for the remainder of the loan are calculated at the new rate.
• The sum of the discounted repayments is the payout figure.