financial math

dont use excel

Tom has a 30-year $150,000 mortgage with an 8% interest rate convertible monthly. Immediately after the 120th payment, he refinances the mortgage. The interest rate is reduced to 6.5%, convertible monthly, and the term is reduced to 20 years (so there are 10 years of payments remaining). He also makes an additional payment of $20,000 at the time of refinancing. Calculate the new monthly payment