A finned-tube cross flow heat exchanger with both fluids unmixed is used to heat water (C_pw = 4.2 kJ/kg · K) from 20°C to 75°C. The mass flow rate of the water is 2.7 kg/s. The hot stream (C_pw = 1.2 kJ/kg · K) enters the heat exchanger at 280°C and leaves at 120°C. The overall heat-transfer coefficient is 160 W/m² · K. Determine 

a. The mass flow rate of the heat stream 

b. The exchanger surface area