Speed of sound, v = 342 m sˆ’1
Echo returns in time, t = 3 s
Distance travelled by sound = v — t = 342 — 3 = 1026 m
In the given time interval, sound has to travel a distance that is twice the distance of the reflecting surface and the source.
Hence, the distance of the reflecting surface from the source=1026/2m = 513 m.