At what fraction of the speed of sound must a sound be reced

At what fraction of the speed of sound must a sound be receding, if the perceived frequency is one third its stationary frequency?

Solution

f_o = f_s (V/V+V_s) can be written as f₀/f_s  = (1/(1+v_s/v)) ; f_s/f₀ = 1+ V_s/V ; (V_s-V)/V = ( f_s/f₀) -2 = 3-2 =1