Sound of an incoming automobile

Two trains separated by $$2\text{ km}$$, travel with the same speed of $$v = 90\text{ km/h}$$ on the same straight track. A man sits at a point $$A$$, located at a distnace of $$1\text{ km}$$ from the track. At the time when the trains are equidistant from point $$A$$, they simultaneously emit a sound signal of the same frequency $$f_0 = 500 \text{ Hz}$$.

Which of the following is the best approximation for the beat frequency hear by the man at $$A$$.

Take the speed of the sound as $$c = 350\text{ m/s}$$.

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