Two identical train whistles, each of frequency 520 Hz, are sounded continuously. Train X moves east at 30 m/s and Train Y moves west at 30 m/s along the same straight track. A 10 m/s wind blows from west to east. A stationary observer stands exactly halfway between the trains. The speed of sound in still air is 340 m/s.
Determine the beat frequency heard by the observer.