An engine approaches a hill with constant speed and where it is at a distance of 1 km blows a whistle whose echo is heard by the driver after 5 seconds. If the speed of the sound is $340 {\mathrm{ms}}^{-1}$. The speed of the engine is

The distance between hill and the engine = 1000m 
echo = 5s
speed of sound = 340 m/s
 

Initially the engine is at  A point when it is received the echo engine is at B point.
$\begin{array}{r}2000-\mathrm{d}=340\times 5\\ \mathrm{d}=300\mathrm{m}\end{array}$
Distance traveled by the engine in 5s is 300
Speed of the engine $=\frac{300}{5}$
=60 m/s