What is an echo? How can you calculate the speed of sound using echo?

A person stands in between two parallel cliffs which are $99 \mathrm{m}$ a part. He fires a gun and hears two successive echoes after $0.2 \mathrm{s}$ and $0.4 \mathrm{s}$. Calculate the Speed of sound.

A man standing in front of a vertical cliff fires a gun. He hears the echo after $3$ seconds. On moving closer to the cliff by $82.5 \mathrm{m}$, he fires again. This time, he hears the echo after $2.5$ seconds. Calculate the distance of the cliff from the initial position of man.