Q:  What quantity will a perfect price discriminating firm facing demand given by P=35-2Q choose if its MC = 5?

A:  For a perfect price discriminating firm, P=MR since it does not have to lower its price to other customers to increase its sales.
        Therefore we have MR=35-2Q. 
        As always the firm sets MR=MC to find its quantity.
        This yields 35-2Q=5 or 30=2Q or Q=15.