On (c): what follows compares the percentage increase in the payment on the 40 year loan to the percentage decrease in duration. (This is the opposite direction of the question, but the percentages work out better.)
Using the answer to (b), out of total payments after 30 years, only 17.48% went to pay down the principal. An increase in the payment from 481 to 507 would be an increase of 5.50%. This represents a percentage increase in payments to principal of 5.50/17.48 = 31.48%. To summarize, a 5.50% increase in total payment amount results in a 31.48% increase in payments to principal, which in turn results in a 25% decrease in duration as the loan is paid off faster.