Answer:
If
the time gap between the original sound and reflected sound received by the listener
is around 0.1 s, only then the echo can be heard.
The minimum distance travelled by the reflected sound wave for the distinctly
listening the echo
= Velocity of sound x Time interval
= 344 x 0.1 = 34.5m
But in this case the distance travelled by the sound reflected from the
building and then reaching to the girl will be (6 + 6) = 12m, which is much
smaller than the required distance. Therefore, no echo can be heard.
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