Suppose that in addition to repairing your car, you can also spend money advertising. If you repair your car for hours and spend A dollars advertising, your benefit (the amount you revive when you sell your car) is B(H,A)=220H-H^2+2A. Both H and A are finely divisible. What is your marginal benefit of repair hours when H=4 and A=100? What is your marginal benefit of advertising dollars at that choice?