What minimum distance is required between the source of sound and the reflecting surface to hear an echo? Give reason.

The minimum distance required between the source of sound and the reflecting surface to hear an echo is 16.5 m.
Speed of sound in air is 330 m s^-1 and the time interval needed to identify two different sounds is 0.1 s, so sound will travel a distance of 330 x 0.1 = 33 m. Since sound has to travel an equal distance in going up to the reflecting surface and in coming back from the reflecting surface, so it must travel nearly $\dfrac{\text{33}}{\text{2}}$ = 16.5 m either way.