A swimming pool is heated by a gas-fired heater that supplies 200,000 Btu/hr to the water. The pool is rectangular, 15 ft by 30 ft, with an average depth of 5 ft. The water circulating system keeps the water temperature quite uniform, even while the heater is operating. The heat loss from the pool has been found to be approximately equal to 5000 Btu/hr for each degree Fahrenheit difference between the temperature of the water and that of the ambient air. Write the differential equation that describes how the water temperature varies with time when the heater is turned on and the ambient air temperature is 50 degrees Fahrenheit.