The driver of a car travelling with speed $30 \,m / s$ towards a hill sounds a horn of frequency $600 \,Hz$. If the velocity of sound in air is $330 \,m / s$, the frequency of reflected sound as heard by driver is ........ $Hz$
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(d)
Velocity of sound $=330 \,m / s$
Frequency of incident sound on wall $\left(f_1\right)=\frac{v}{v-v_s} \times f$
Frequency of sound observed by driver $\left(f_2\right)=\frac{v+v_s}{v} \times f_1$
or $f_2=\frac{v+v_s}{v-v_s} \times 600$
or $f_2=\frac{360}{300} \times 600$
or $f_2=720 \,Hz$
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