A train is moving on a straight track with speed $20\ ms^{-1}$. It is blowing its whistle at the frequency of $1000\ Hz$. The percentage change in the frequency heard by a person standing near the track as the train passes him is ( speed of sound $=320$ $ms^{-1}$ ) close to .... $\%$
A$12$
B$18$
C$24$
D$6$
JEE MAIN 2015, Medium
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A$12$
a $f_{1}=f\left[\frac{v}{v-v_{s}}\right]=f \times \frac{320}{300} \mathrm{Hz}$
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