Add-on interest requires us to calculate the total interest to be paid over 4 years. After deducting the down payment there is $16500-1500=$15000 to pay and the total interest is 4×0.0725×15000=$4350 to be paid over 48 months. That’s $90.625 per month for the interest alone. This is added to 15000/48=$312.50 for the principal (the loan). The monthly payment is therefore 312.50+90.625=$403.125 or, rounded up, $403.13.
Another way to do the calculation is to add the principal to the interest then divide by the number of monthly payments: (15000+4350)/48=19350/48=$403.13 after rounding.