A train moving with a speed of 20.9 m/s sounds a 103 Hz horn. What frequency is heard by an observer standing near the tracks as the train approaches

observed frequency =  109.68 Hz

Explanation:

given data

speed u = 20.9 m/s

frequency f = 103 Hz

solution

we consider here speed of sound (v)  is 343 m/s

so observed frequency is express as

observed frequency = f ×     ...............1

put here value and we get

observed frequency =  103 × \frac{343}{343-20.9}

observed frequency =  109.68 Hz

b would be the answer