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**1. Homework Statement**

Two trains emit whistles of the same frequency, 282 Hz. If one train is at rest and the other is traveling at 45 km/h away from an observer at rest, what beat frequency will the observer detect?

**2. Homework Equations**

**3. The Attempt at a Solution**

I used listener behind formula and got an answer of 544 Hz because i doubled the Frequency source. The answer is wrong, please help me!Thanks!