Two trains emit whistles of the same frequency, 274 Hz. If one train is at rest and the other is traveling at?

Two trains emit whistles of the same frequency, 274 Hz. If one train is at rest and the other is traveling at 30 km/h away from an observer at rest, what beat frequency will the observer detect?
________Hz

Ok, let's assume that the observer is on the train at rest, so :

Using and remembering Doppler effect :

fL = (v + vl / v)*Fs

fL = frequency detected by the observer

Fs = 274 Hz

vl = 30*5/18 = 25/3 m/s

fL = ( 340 + 25/3 / 340)*274

fL = 280.7 Hz

Hope that helps