In a counter-flow heat exchanger, water is heated at the rate of 1.5kg/s from 40 °C to 80 °C by an oil entering at 120 °C and leaving at 60 °C. The specific heats of water and oil are 4.2kJ/kg/K and 2kJ/kg.K respectively. The overall heat transfer coefficient is 400 W/m².K. The required heat transfer surface area (in m²) is