Echo can be heard if the gap between the original sound and reflected sound received by the listener is around 0.1 sec.
Speed of sound in air, (v) = 344 ms -1
Sound Velocity × time interval
= 344 × 0.1
= 34.4 m
Here sound reflects from the building and then reaches the girl which is much smaller than the required distance. Hence echo cannot be heard.
However, in this situation, the sound reflected from the building and then reaching the girl will travel a distance of (6 + 6) = 12 m, which is significantly less than the required distance. As a result, there is no echo.