To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1 s.
If we take the speed of sound to be 344 m/s at a given temperature, say at 2o C in air, the sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1 s. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least (344m/s) * 0.1s = 34.4 m.
Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m.
But in this case, the distance traveled by the sound reflected from the building and then reaching to the girl will be (6 + 6) = 12 m. That is, the minimum distance of the obstacle from the source of sound is 6 m, which is much smaller than the required distance (17.2 m). Therefore, no echo can be heard.
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