or

What is the minimum distance between a source of the sound and a reflector to hear an echo?

Let S be the distance between the source of sound and reflector.
V be the velocity of sound in air.
t be the total time taken by the sound to travel from source to reflector 
and back to the source.

        2S = Vt
or,    S   = (Vt)/2

We can hear distinct sounds if they reach our ear after an interval of 0.1s or more
That is, t = 0.1s and V = 340 ms-1

Therefore, S = (340 x 0.1)/2 = 17 m.
This is required minimum distance to hear an echo.