An echo was heard after 3s.What is the distance of the reflecting surface from the source,given that the speed of sound is 342m/s?

Speed of sound = 342m/s

Echo returns in time, t =3s

Distance travelled by sound ,= v x t

                                             = 342 x 3

                                             =1026 m

As the sound has to travel twice the distance from the source to and the reflecting surface during an echo.
:. Distance of the reflecting surface from the source

                                         = 1026/2 =513 m