A car approaches a stationary listener at a speed of 72 km/h and emits a tone of 600 Hz. At the same time, another car moves away from the stationary listener at a speed of 54 km/h and emits a tone of unknown frequency. What is this unknown frequency if the stationary listener hears a tone of the same frequency from both cars? What frequency of the tone is heard by the stationary listener? The speed of sound is in air 340 m/s.
