A counter flow double-pipe heat exchanger

A counter flow double-pipe heat exchanger is to heat water from 20°C to 80°C at a rate of 1.2 kg/s. The heating is to be accomplished by geothermal water available at 160°C at a mass flow rate of 2 kg/s. The inner tube is thin-walled and has a diameter of 1.5 cm. If the overall hear transfer coefficient of the heat exchanger is 640 W/m²K. Determine the length of the heat exchanger required to achieve the desired heating.