what is an epoch ?

Explanation

(i) An echo is the sound reproduced due to the reflection of the original sound from various rigid surface such as wall, ceilings, surfacew of mountains, etc.
(ii)(i)The persistence of hearing for human ears is 0.1 second. This means that we can hear two sound waves clearly, if the time intervel between the two sounds is atleast 0.1 s. Thus, the minimum time gap between the original sound and an echo must be 0.1 s.
(ii)The above criterion can be stisfied only when the distance between the source of sound and the reflecting surface would satisfy the following equation.
Velocity =`("Distance travelled by sound")/("Time taken")`
`v=(2d)/(t)`
d=`(vt)/(2)`
Since,t=0.1 second, then d`=(vxx0.1)/(2)=(v)/(20)`