Non-excel solution please

Tom and Marie have a 30-year $150, 000 mortgage with an 8% interest rate convertible monthly. Immediately after the 120th payment, they refinance the mortgage. The interest rate is reduced to 6.5%, convertible monthly, and the term is reduces to 20 years (so there are 10 years of payments remaining). They also make an additional payment of $20, 000 at the time of refinancing. Calculate their new monthly payment.