# A driver in a car approaching a vertical wall notices that the frequency of his ear horn has changed from 440 Hz to 480  Hz when it gets reflected from the wall. If the speed of sound in air is 345 m/s, then the speed of the car is: Option: 1 36km/hr  Option: 2 18 km/hr Option: 3 24 km/hr   Option: 4 54 km/hr

$\begin{array}{l} f_{0}=\left(\frac{v+u}{v-u}\right) \times f_{s} \\ \\ Or, 480=\left(\frac{v+u}{v-u}\right) \times 440 \\ \\ Or, \left(\frac{48}{44}\right)=\frac{v+u}{v-u}=\frac{345+u}{345-u} \\ \\ Or, 1.0909 \times 345-1.0909 u=345+u \\ \\ Or, u=15 m / s=54km/hr \end{array}$

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