\(f _{1}=\) frequency of the car w.r.t. the hill
\(f_{1}=\left(\frac{v}{v-v_{ s }}\right) f=\left(\frac{330}{320}\right) \times 320=330 Hz\)
\(f _{2}=\) Frequency of the sound reflected by hill w.r.t. the car \((echo)\)
\(f_{2}=\left(\frac{v+v_{0}}{v}\right) f_{1}=\frac{(330+10)}{330} \times 330=340\,Hz\)