The outlet temperature (in K) of the preheated air is

Question

A balanced counter-flow heat exchanger has a surface area of 20 m² and overall heat transfer coefficient of 20 W/m²-K Air. (c_p = 1000 J/kg-K) entering at 0.4 kg/s and 280 K is to preheated by the air leaving the system at 0.4 kg/s and 300 K. the outlet temperature (in K) of the preheated air is                                

      (a) 290                            (b) 300                         (c) 320                            (d) 350