What is the minimum distance of the obstacle from the source of sound for hearing distinct echo ?

For hearing distinct echo, the minimum distance of the obstacle from the source of sound should be ....... m. (10.2, 34.4, 17.2)

What is the minimum distance needed for an echo?

What is the minimum distance needed for an echo?

An echo is heard after 5 seconds of the production of sound which moves with a speed of 340 m/s. What is the distance of the mountain from the source of sound which produced the echo?