An object is kept in between two walls 50 metre apart. If first echo is heard after 0.1 Second, when would you hear the second echo?

Dear student,
since the object is in the middle of both walls so it is 25m away from each wall.

So for first echo is heard when the sound is reflected back from the wall at one of walls. So here total distance travelled by the sound wave is 25+25=50m
And time taken is 0.1 sec
Therefore velocity of sound here will be (distance / time)=50/0.1 = 500 m/s
Second echo will be heard when the sound wave comes back after being reflected from both the walls.
So the total distance travelled by the sound will be
Time taken= distance/velocity=100/500= 0.2 sec
Hence next echo will be heard after 0.1 sec.

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