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**1. Homework Statement**

You are standing on a train station platform as a train goes by close to you. As the train approaches, you hear the whistle sound at a frequency of f1 = 92 Hz. As the train recedes, you hear the whistle sound at a frequency of f2 = 79 Hz. Take the speed of sound in air to be v = 340 m/s.

Find the numeric value, in hertz, for the frequency of the train whistle fs that you would hear if the train were not moving.

**2. Homework Equations**

f obs = f s (v +- vobs / v +- vs)

**3. The Attempt at a Solution**

I tried to find fs by fs = f obs / (v +- vobs / v +- vs), but since nobody is moving I just get 1 in parenthesis, and don't know what to set f obs to. The correct answer is 85 Hz, I just have no clue how to get to that point.