An echo is returned in `3 s`. What is the distance of the reflecting surface from the source, given that the speed of sound is `342 m//s`.

Explanation

Speed of sound, `v = 342 ms^(-1)`
Echo returns in time, t = 3 s
Distance travelled by sound `= v xx t = 342 xx 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 / 2 m = 513 m.