A Train Moving at a Constant Speed Is Passing a Stationary

Multiple Choice

A train moving at a constant speed is passing a stationary observer on a platform. On one of the train cars, a flute player is continually playing the note known as concert A (f = 440 Hz) . After the flute has passed, the observer hears the sound with a frequency of 415 Hz. What is the speed of the train? The speed of sound in air is 343 m/s.

A) 7.3 m/s
B) 12 m/s
C) 21 m/s
D) 37 m/s
E) 42 m/s