A drone aeroplane is flying horizontally at a constant height of 4000 ft above a fixed radar tracking station. At a certain instant the angle of elevation theta is 30 degrees and is decreasing, and the speed of the drone aeroplane is 300 mi/h. How fast is the distance between the aeroplane and the radar station changing at this instant? Express the rate in units of ft/s. Use 1 mi=5280 ft