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. `32 m//s`
B. `27.5 m//s`
C. `60 m//s`
D. `30 m//s`

Answer

D

Explanation

Let after 5 sec engine at point C
`t=(AB)/330+(BC)/330`
`5=(0.9 times1000)/330+(BC)/330 thereforeBC=750 m`
Distance travelled by engine in 5 sec
`5=(0.9 times1000)/330+(BC)/330 thereforeBC=750 m`
Distance travelled by engine in 5 sec
=900m-750m=150m
Therefore velocity of engine `=150 m/5 sec=30 m//s`