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You (a stationary observer) are standing by the railroad tracks and hear a frequency of 1000 Hz as the train approaches... as the train goes away, the frequency changes to 800 Hz. Knowing that the speed of sound is 340 m/s, how fast is the train moving?

Now, I understand that the observed frequency seems different (larger) as the train approaches, and smaller when the train leaves. However, I cannot figure out how to find the actual frequency from the source in order to calculate the speed of the train. It seems to me that I have 2 unknowns. The equation I was trying to use was:

f (observed) = f (source) * (speed of sound/

speed of sound -

speed of train)

Any help would be appreciated!