Part (a): Mortgage amount to borrow
15% down payment will be made. Therefore, only 85% (100-15) will be borrowed.
That is,
Amount to borrow, P = $125,340*85/100 = $109,539
Part (b): Affordability
Gross annual income = $64,570
Less fixed expenses = (1-0.28)*64,570 = $46,490.40
If, on worse scenario, the loan is to be paid in four or so years, then net income can support that. Now, the loan is to be paid in 20 years and thus this is affordable.
Part (c): Monthly payments
Monthly payments, M = P[r(1+r)^n]/[(1+r)^n-1]
Where, r = 0.0375/12; n = 20*12 = 240 months
Substituting;
M = 109,539[0.0375/12(1+0.0375/12)^240]/[(1+0.0375/12)^240-1] = $640.44
Part (d): Total payment for the house
Total payment = Down payment + (Monthly payments*20*12) = (125,340 - 109,539) + (640.44*20*12) = 15801 + 153,705.60 = $169,506.60
Part (e): Total interest to be paid
Interest to be paid = Total payment - Cost of the home = 169,506.60 - 125,340 = $44,166.60