While waiting for Stan Speedy to arrive on a late passenger train, Kathy Kool notices beats occurring as a result of two trains blowing their...

While waiting for Stan Speedy to arrive on a late passenger train, Kathy Kool notices beats occurring as a result of two trains blowing their whistles simultaneously. One train is at rest and the other is approaching her at a speed of 9.05 km/h. Assume that both whistles have the same frequency and that the speed of sound is 344 m/s. If Kathy hears a beat frequency at 7.92 Hz,what is the frequency of the whistles? Answer in units of Hz.