A person is standing in front of a wall and makes a sound. What should be the normal minimum distance between the person and the wall , so that the person hears echo?

Q. A person is standing in front of a wall and makes a sound. What should be the normal minimum distance between the person and the wall , so that the person hears echo?
Answer: 17 meters
Notes: To hear an echo, the minimum distance between a wall and the source of sound is 17m. The minimum interval is 0.1 sec.