Water at 1.2 kg/s enters the tubes of a two-shell-pass, four tube-pass heat exchanger at 17C and leaves at 42C. It cools 0.9 kg/s of air entering the shell at 250C with U=532 W/m2 -K. Determine: (a) the exit air temperature; (b) the area of the heat exchanger; and (c) the exit temperature if, after some time, the tubes become fouled with Rf =0.0005 m2 -K/W.