ADD-ON LOANS vS SIMPLE INTEREST LOANS. You are borrowing $500,000 on a 30 year, 5.5% interest loan to buy a property. 18a) What will the total interest be if it was an add-on interest loan? 500,000305.5%=$825,000 18b) How much would the monthly payment be (for 30 years) if it was an addon interest loan? 825000/3012=$2291.67 sonal Finance f. Evans KE HOME 2 18c) What will the total interest be if it was a simple interest amortized loan? 5.5%/12=0.00463012=360 18d) How much would be the monthly payment (for 30 years) if it was a simple interest amortized loan