A shell-and-tube heat exchanger is used to cool oil (C_H = 2.2kJ/kg · K) from 110°C to 65°C. The heat exchanger has two shell passes and four tube passes. The coolant (C_c = 4.20 kJ/kg · K) enters the shell at 20°C and leaves the shell at 42°C. For an overall tube-side heat-transfer coefficient of 1200 W/m² · K and an oil flow of 11 kg/s. determine 

a. The coolant mass flow rate 

b. The required surface area in the exchanger