The driver of a car travelling with speed $30\text{m/sec}$ towards a hill sounds a horn of frequency $600\text{Hz}$. If the velocity of sound in air is $330\text{m/s}$, the frequency of reflected sound as heard by driver is:

This is a two-step Doppler effect problem:Source moves towards the stationary hill (observer):Frequency received by the hill ($f^{\prime }$):
$$f^{\prime }=f\left(\frac{v}{v-v_s}\right)=600\left(\frac{330}{330-30}\right)=600\left(\frac{330}{300}\right)=660\text{Hz}$$
Hill acts as a source, driver (observer) moves towards the reflected sound:Frequency heard by the driver ($f^{\prime \prime }$):
$$f^{\prime \prime }=f^{\prime }\left(\frac{v+v_o}{v}\right)=660\left(\frac{330+30}{330}\right)=660\left(\frac{360}{330}\right)=2\times 360=720\text{Hz}$$