Astudent went to a hill station early in the morning, he could hear the echo of his clap after 0.1 second. when he went to the same place in the afternoon, he could not hear the echo at all, explain the reason for his changed behaviour.

See the explanation.

Explanation:

The speeds of sound depend upon the temperature of the medium. As the temperature increased in the afternoon, the speed of sound increases. Then the time taken by reflected sound will be less than 0.1 sec in the afternoon. And to hear an echo the time gap between an original sound and reflected sound must be at least 0.1 sec. That is why the student could not hear the echo at all in the afternoon.

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answer: 31, 5556. 952 seconds