How much should minimum distance be between the source of sound and reflecting surface, so that an echo can be heard clearly?

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?

A worker was working on the rail track. A boy at a distance holds his ear near the rail track. This boy was able to hear sound of the blow of workman twice. It is because of :

An astronaut cannot hear his companion at the surface of the moon because –

When the same tone is played on a sitar and a fute, the sound produced can be distinguished from each other because of the diference in :

In which one among the following mediums would the speed of sound be maximum ?