A classroom that normally contains 40 people is to be air conditioned with window air conditioning units of 5 kW cooling capacity. A person at rest may be assumed to dissipate heat at a rate of about 360kJ/h. There are 10 lightbulbs in the room, each with a rating of 100 W. The rate of heat transfer to the classroom through the walls and the windows is estimated to be 15,000 kJ/h. If the room air is to be maintained at a constant temperature of 21 degree Celsius , determined the number of window air conditioning units required.