Shirley borrowed $1,500 to buy some new furniture, but she doesnt know which interest calculation method was used on her purchase. Calculate monthly payments assuming a 1 year repayment period and 14% interest. Now assume the store uses add-on method of interest calculation, calculate the monthly payment and total cost assuming a 1 year repayment period but only a 12% interest charge. Why at a lower interest rate, does the add-on method of interest calculation result in higher monthly payments?