A car sounding a horn of frequency $ 1000 Hz$ passes an observer. The ratio of frequencies of the horn noted by the observer before and after passing of the car is $11 : 9$. If the speed of sound is v, the speed of the car is
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${n_{Before}} = \frac{v}{{v - {v_c}}}n$ and ${n_{After}} = \frac{v}{{v + {v_c}}}\,n$
$\frac{{{n_{Before}}}}{{{n_{After}}}} = \frac{{11}}{9} = \left( {\frac{{v + {v_c}}}{{v - {v_c}}}} \right)$
==> ${v_c} \Rightarrow \frac{v}{{10}}$
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