The driver of a bus approching a big wall notices that the frequency of his bus's horn changes from 420Hz to 490Hz when he hears if after it gets reflected from the wall. Find the speed of the bus if speed of the sound is 330ms-1 Option: 1 91 kmh-1 Option: 2 81 kmh-1 Option: 3 61 kmh-1 Option: 4 71 kmh-1

\begin{aligned} &\text { Let the velocity of car be } u\\ &\text { Then } 490=\frac{v+u}{v-u} 420 \text { or } \end{aligned}

$\begin{array}{l} \frac{330+u}{330-u} \times 420=490 \\ \\ \Rightarrow \quad \frac{330+u}{330-u}=\frac{7}{6} \\ \\ \Rightarrow \quad 13 u=330 \\ \\ \Rightarrow \quad u =\frac{330}{13}=25.38 \mathrm{~m} / \mathrm{s} \\ \\=91 \mathrm{~km} / \mathrm{h} \end{array}$

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