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General
Easy
Question
An engine approaches a hill with a constant speed. When it is at a distance of 0.9 km it blows a whistle, whose echo is heard by the driver after 5 sec. If speed of sound in air is 330 m/s, the speed of engine is
- 10 m/s
- 20 m/s
- 30 m/s
- 40 m/s
The correct answer is: 30 m/s
If the speed of engine is v, the distance traveled by engine in 5 sec will be 5v, and hence the distance traveled by sound in reaching the hill and coming back to the moving driver = 900 + (900 – 5v) = 1800 – 5v
So the time interval between original sound and it’s echo 
Þ v = 30 m/s
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