Question

a man us walking towards vertical cliff at constant speed u m/s and fires a gun and hears the echo after t seconds if it's distance is d find the velocity of sound in air​

Answer 1

He hears the echo after t time. So the velocity of the sound to travel a distance of 2d is =2d/t.  

Now the man is also moving towards the cliff with a velocity u so the actual velocity of the sound in the air is =(  2d /t−u) m/s