While waiting for Stan Speedy to arrive on a late passenger train, Kathy Kool notices beats occurring as a result of two trains blow- ing their whistles simultaneously. One trainis at rest and the other is approaching her at a speed of 19.2 km/h. Assume that both whistles have the same frequency and that the speed of sound is 344 m/s. If Kathy hears beat frequency at 3.81 Hz, what is the frequency of the whistles? Answer in units of Hz