David buys a new car and finances it with a loan of 20,000. He will make monthly payments of 420.50 starting in one month with a smaller drop payment (last payment) to pay off the loan. Payments are calculated using an annual nominal interest rate of 6% convertible monthly. Immediately after the 15th payment, he repays an additional 1000 and then refinances the loan to pay off the remaining balance with 24 monthly payments starting one month later. This refinanced loan uses an annual nominal interest rate of 4.8%, convertible monthly. Calculate the amount of the new monthly payment