Question

An observer traveling with a constant velocity of 20m/s passes close to a stationary source of sound and noticed that there is a change of frequency of 50Hz as he passes the source....what is the frequency of the source(speed of sound in air=340m/s)

Answer 1

$$f_L=\frac{v+v_L}{v+v_S}f_S\\f_S+50=\frac{340+20}{340+0}f_S\\ f_S=850\ Hz$$