What is the minimum distance (in metres ) required to hear an echo ?

Explanation

An echo can be heard only if the reflected sound reaches our ears after a gap of at least one-tenth of a second, .i.e., 0.1 sec. If it comes before that, the original sound and the reflected sound will overlap, ,and a person will not be able to hear the sound.
If the weed of sound is taken as 340 m/s, then in 0.1 sec, distance travelled by sound= speed `xx` time.
`=(340xx0.1)m=34m`

Since for an echo, the sound produced will travel to the reflecting wall and return as echo to the observer after reflection, total distance covered by sound, before being heard as echo
= twice, the distance between the observer and the reflecting wall.
Hence,minimum distance between the observer and the reflecting wall `=1/2xx34m=17m`