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.