Engine oil is cooled from 80 °C to 50 °C in a single-pass counter-flow concentric-tube exchanger by water at 20 °C (thin inner tube 2.5 cm, 0.08 kg/s); oil 0.16 kg/s; h_{w} = 1000, h_{oil} = 80 W/m^{2}·K; fouling 0.00018 m^{2}·°C/W on each side; c_{p,w} = 4180, c_{p,oil} = 2090 J/kg·K. Calculate (a) U (W/m^{2}·K), (b) the area (m^{2}) and (c) the length (m).