An observer and a source of sound are initially both stationary, and the observer measures the frequency of the sound to be f1. Then the source moves toward the observer with a velocity of 15.0 m/s due east (with the observer still at rest), and the observer measures a new frequency f2. Finally, the observer moves toward the source (which is still approaching at 15.0 m/s) with a velocity of 25.0 m/s due west, and hears a new frequency f3 that differs from f1 by 87.0 Hz. What are the frequencies f1, f2 and f3? You should take the speed of sound to be 343 m/s.