A police cruiser sets up a novel radar speed trap, consisting of two transmitting antennas at the edge of a main north-south road. One antenna is 2.0 m [W] of the other. The antennas, essentially point sources of continuous radio waves, are fed from a common transmitter with a frequency of 3.0x10 Hz. The trap is set for cars travelling south. A motorist drive a car equipped with a "radar detector" along an east-west road that crosses the main road at a level intersection 100 m [N] of the radar trap. The motorist hears a series of beeps upon driving west through the intersection. The time interval between successive quiet spots is 0.20 s as the car crosses the main road. How fast is the car moving? [25 m/s]