\(f_A=\frac{v}{v-v_s} f_0 \quad\) (Frequency of approach)

\(f_R=\frac{v}{v+v_s} f_0 \quad\) (Frequency of departing)

\(\Delta f_A=\left(\frac{v}{v-v_s}-1\right) f_0\)

\(\Delta f_A=\frac{v}{v-v_s} f_0\)

\(\Delta f_R=\left(1-\frac{v}{v+v_s}\right) f_0\)

\(\Delta f_R=\frac{v}{v+v_s} f_0\)

\(\frac{\Delta f_A}{\Delta f_R}=\frac{v+v_s}{v-v_s}=\frac{3}{2}\)

\(2 v=2 v_s=3 v-3 v_s\)

\(5 v_s=v\)

\(v_s=\frac{v}{5}\)