If the effectiveness of the heat exchanger is

Question

Water (C_p = 4.18 kJ/kg. K) at 80⁰C enters a counter flow heat exchanger with a mass flow rate of 0.5kg/s. Air (C_p = 1kJ/kg. K) enters at 30⁰C with a mass flow rate 2.09 kg/s. If the effectiveness of the heat exchanger is 0.8, the LMTD (in ⁰C) is                                               

      (a) 40                              (b) 20                           (c) 10                              (d) 5