A person driving car at a constant speed of $15\,m / s$ is approaching a vertical wall. The person notices a change of $40\,Hz$ in the frequency of his car's horn upon reflection from the wall. The frequency of horn is ............ $Hz$. (Given : Speed of sound : $330\,m / s$ )
JEE MAIN 2023, Diffcult
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$\text { Frequency of reflected sound }=\left(\frac{ v + v _{ c }}{ v - v _{ c }}\right) f _0$
$f =\left(\frac{330+15}{330-15}\right) \times f _0$
$=\frac{345}{315} f _0$
$\frac{345}{315} f _0- f _0=40$
$\frac{30}{315} f _0=40$
$f _0=\frac{4 \times 315}{3}=420\,Hz$
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