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A car is moving with a speed of $72 \mathrm{~km} / \mathrm{h}$ towards a hill. Car blows horn at a distance of $1800 \mathrm{~m}$ from the hill. If echo is heard after $10 \mathrm{~s}$, the speed of sound (in $\mathrm{m} / \mathrm{s}$ ) is
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Verified Answer
The correct answer is:
$340$
The speed of the car is $72 \mathrm{~km} / \mathrm{h}$
$$
=72 \times \frac{5}{18}=20 \mathrm{~m} / \mathrm{s}
$$
The distance travelled by car in $10 \mathrm{~s}$
$$
=10 \times 20=200 \mathrm{~m}
$$
Hence, the distance travelled by sound in reaching the hill and coming back to the moving driver
$$
\begin{aligned}
& =1800+(1800-200) \\
& =3400 \mathrm{~m}
\end{aligned}
$$
So, the speed of sound $=\frac{3400}{10}=340 \mathrm{~m} / \mathrm{s}$
$$
=72 \times \frac{5}{18}=20 \mathrm{~m} / \mathrm{s}
$$
The distance travelled by car in $10 \mathrm{~s}$
$$
=10 \times 20=200 \mathrm{~m}
$$
Hence, the distance travelled by sound in reaching the hill and coming back to the moving driver
$$
\begin{aligned}
& =1800+(1800-200) \\
& =3400 \mathrm{~m}
\end{aligned}
$$
So, the speed of sound $=\frac{3400}{10}=340 \mathrm{~m} / \mathrm{s}$
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