The phenomenon due to which repetition of sound is heard after reflection from a distant object, after the original sound from a given source dies out is called an echo.
Let‘t’ be the time after which an echo is heard, ‘d’ is the distance between the source of sound and reflecting body and 'Y is the speed of sound. Then in time t, distance travelled by sound = 2d
Hence, in one second distance travelled by sound = 2 d/t.
But distance travelled by sound in one second = Speed of sound (V)
Therefore, V = 2 d/t
or d = Vx t/2