$4-18$ In a falling head permeability test on fine sand, the sample is $70mm$ diameter and $174mm$ length and the standpipe $17mm$ diameter. A stop watch is started when the head was $1m$ and stopped when head was $0\mathrm{.}35m$ and reads $3\mathrm{.}8$min. The test is repeated for a drop from $0\mathrm{.}3m$ to $0\mathrm{.}1m$ and the time is $7\mathrm{.}75$min. The coefficient of permeability of the soil $(6dp)$ is $(\frac{m}{s})$ :