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 seconds. If the speed of sound in air is 330 m/s, then the speed of the engine is :

Options

A. 80 m/s
B. 30 m/s
C. 300 m/s
D. 360 m/s

Answer

B

Explanation

If the speed of the engine is v the distance covered by engine in 5 sec is 5v
The distance travelled by sound in reaching in hill and coming back to the moving driver
`= 900 + (900 - 5v)`
`= 1800 - 5v`
Time interval between original sound and echo is
`5 = (1800 - 5v)/(330) :. 1800 - 5v = 1650`
`:. v = 30 m//s`