**Solution:**

Given,

Frequency of sound (f) = 150 kHz = 150000 Hz

Velocity of observer, \(u_o=120km/hr=\frac{120\times1000}{3600}m/s=33.33m/s\)

Velocity of sound (v) = 340 m/s

Apparent frequency (f') = ?

We have, the apparent frequency as observed by the truck is,

\(f'=\frac{v+u_o}v\times f=\frac{340+33.33}{340}\times150000=164704.41Hz=164.7\;kHz\)

Again, the truck acts as a source with frequency f' and the observer is the detector. So, the frequency of wave reflected back to the detector is

\(f'=\left(\frac v{v-v_s}\right)\cdot f=\left(\frac{340}{340-33.34}\right)\times164.7=182.6\) kHz