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. `30ms^(-1)`
B. `60ms^(-1)`
C. `32ms^(-1)`
D. `27.5ms^(-1)`

Answer

B

Explanation

Let v be the speed of the engine.
Distance travelled by the engine in 5 s is 5v.
The distance travelled by sound in reaching the hill and coming back to the moving driver
`=900+(900-5v)=(1800-5v)`

Time interval between the original sound and its echo
`(1800-5v)/(330)=5s` (given)
or `1800-5v=1650`
or `5v=1800-1650orv=30ms^(-1)`