Can anyone please help me with part B? Would greatly appreciate it!

4. FINANCING A HOME-The Millers are planning to buy a home in the near future and estimate that they will need a 30-year fixed-rate mortgage for $240,000. Their monthly payment P (in dollars) can be computed by using the formula 20,000r 1-(12-360 where r is the interest rate per year a. Find the differential of P. If the interest rate increases from the present rate of 5%/year between now and the time the Miller's decide to secure the loan, approximately how much more b. will their monthly mortgage payment be? How much more wil it be if the interest rate increases to 5.3%/year? To 5.4%/year? To 5.5%/year