A 15 cm×20 cm hot surface at 85°C is to be cooled by attaching 4 cm long aluminum (k =237 W/m °C) fins of 2 mm×2-mm square cross section. The temperature of surrounding medium is 25°C and the heat transfer coefficient on the surfaces can be taken to be 20 W/m2 °C. If it is desired to triple the rate of heat transfer from the bare hot surface, determine the number of fins that needs to be attached