Question

Refer the attached picture. ​

Answer 1

according to question, police man detects 10% drop in the pitch when the car crosses him. So, the apparent frequency when the car moves away from him is 90% of the apparent frequency when the car approaches him.

now use Doppler's effect of sound, 

therefore , when car approaches him, 

\nu_1=\nu_0\left(\frac{V}{V-V_s}\right)ν1=ν0(V−VsV) 

The apparent frequency , when the car moves away 

\nu_2=\nu\left(\frac{V}{V+V_s}\right)ν2=ν(V+VsV) 

from question, 

v_2=0.9v_1v2=0.9v1 

so, \frac{V}{V+V_s}=0.9\frac{V}{V-V_s}V+VsV=0.9V−VsV 

9(V+Vs) = 10(V - Vs) 

19Vs = V

Vs = V/19 = 330/19 = 17m/s