Question

why will an echo not be heard when the distance between the source of sound and the reflecting surface is 10m

Answer 1

Hi
hope this helps
The minimum distance to hear an echo is scientifically proven to be 17 meter of average
and therefore no echo will be heard at a distance of 10 meter

Answer 2

the echo will not be heard because the minimum distance required in a temp. of 20°c is 17.2
$2d = s \times t$
$d = (344 \times 0.1) \div 2$
ans...is 17.2