Question 1200996
calculator at <a href = "https://arachnoid.com/finance/" target = "_blank">https://arachnoid.com/finance/</a is used to solve this.
at 5% per year compounded monthly, the payment required is 659.96.
at 4.2% per yer compounded monthly, the payment requred is 616.57.
the difference is 43.39.
payments are made at the end of each month.
240 payments of 659.96 = 158,390.4
the interest is that minus 100,000 = 58,390.4
240 payments of 616.57 = 147,976.8
the interest is tht minus 100,000 = 47,976.8
the difference in the interest payments is 58,390.4 minus 47,976.8 = 10,413.6
that's how much less in interrest is expected to paid after refinancing.
here are the results from the calculator.


<img src = "http://theo.x10hosting.com/2023/031501.jpg">


<img src = "http://theo.x10hosting.com/2023/031502.jpg">


the yearly interest rate is divided by 12 to get the monthly interest rate.
all payments are made at the end of each month.
inputs are everything except payment.
output is payment.
payments are negastive because it's money spent.