An echo is returned in 6 seconds. What is the distance of the reflecting surface from the source?  [Speed of sound is 342 m/s]

Given,

Speed of sound = 342 m/s

Time taken by the echo to return = 6 sec

In 6sec, sound has to travel twice the distance between the source and the reflecting surface. 
Distance traveled in 6 sec is

2d = speed x time

2d = 342 x 6

2d = 2052 m
Therefore, the distance of the reflecting surface from the source, $\Rightarrow \mathrm{d}=\frac{2052}{2\mathrm{}}$
$\mathbf{\Rightarrow }\mathbf{d}\mathbf{=}\mathbf{1026}\mathbf{ }\mathbf{m}$

The distance of the reflecting surface from the source is 1026m.