A 15 cm x 20 cm hot surface at 85 degrees is to be cooled by attaching 4 cm long aluminum (k= 237 w/m degrees C) fins of 2 mm x 2 mm square cross section. The tempurature of surrounding medium is 25 degrees C and the heat transfer coefficent of the surfaces can be taken to be 20 W/m^2 degrees 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.