A 30-L electrical radiator containing heating oil is placed in a 50 m^3 room. Both the room and the oil in the radiator are initially at 10degC. The radiator with a rating of 1.8kW is now turned on. At the same time, heat is lost from the room at an average rate of 0.35 kJ/s. After some time, the average temperature is measured to be 20degC for the air in the room, and 50degC for the oil in the radiator. Taking the density and the specific heat of the oil to be 950 kg/m^3 and 2.2 kJ/kg*degC, respectively, determine how long the heater is kept on. Assume the room is well sealed so that there are no air leaks.