Define echo. Establish a mathematical relation between speed of sound, distance of reflecting body from source of sound, and time for echo.

(N/A) An echo is the phenomenon where the repetition of sound is heard due to reflection from a distant object after the original sound from the source has died out.
Let $t$ be the time taken to hear the echo. Let $d$ be the distance between the source of sound and the reflecting body, and $V$ be the speed of sound.
Since the sound travels from the source to the reflecting body and back to the observer, the total distance covered by the sound is $2d$.
Using the formula: $\text{Speed} = \frac{\text{Total Distance}}{\text{Total Time}}$
We have: $V = \frac{2d}{t}$
Rearranging the formula to find the distance $d$, we get: $d = \frac{V \times t}{2}$