A man who is standing at a point X between two parallel walls (as shown in the diagram) fires a starting pistol.
He hears the first echo after 0.6s and another one after 0.8s. How long after firing the pistol will he hear the next echo?
View video tutorial to visualise the distance travelled.
Time taken to hear next echo = 0.6 + 0.8 = 1.4s
The distance travelled by sound (left and right) from X is the same during 3rd and 4th echos.
Hence 3rd and 4th echos actually coincide. And time taken is 0.6 + 0.8 = 1.4s.