The frequency of echo will be _______ Hz if the train blowing a whistle of frequency 320 Hz is moving with a velocity of 36 km/h towards a hill from which an echo is heard by the train driver. Velocity of sound in air is 330 m/s.

$\begin{array}{l}{\mathrm{v}}_{\mathrm{s}}=36\times \frac{5}{18}=10\mathrm{m}/\sec \end{array}$

The frequency that reaches the hill will reflect, and this will act as a source to the driver in train.

Apparent frequency that reaches the hill $=\frac{330}{330-10}\left(320\right)=330\mathrm{Hz}$, this will act as source for driver.

Apparent frequency heard by the driver in train $=\frac{330+10}{330}\left(330\right)=340\mathrm{Hz}$