Answer:
The Excel function that can be used to calculate the maximum purchase price you can afford based on the given information is the "PV" (Present Value) function.
Assuming that you have a maximum budget of $X to use as a down payment, the formula to calculate the maximum purchase price you can afford would be:
=PV(3.5%/12, 3012, X0.8)
In this formula, the first argument (3.5%/12) represents the monthly interest rate (since the loan is for 30 years), the second argument (3012) represents the total number of months in the loan term, and the third argument (X0.8) represents the present value of the loan (i.e., the amount you can borrow based on your down payment).
Note that the 0.8 factor is used to represent the 20% down payment required by the lender.