Aroom in a house is maintained at 25 °c by a heat pump. the air outside is at 9 °c. heat is being lost out a 4.5 mm thick single-pane window which is 0.5 m high by 1 m wide. if the heat transfer coefficients at the inside and outside surfaces of the window are 12 and 30 w m-2 k-1 respectively and the thermal conductivity of the window glass is 0.78 w m k-1, calculate the rate of heat loss through the window.