A laboratory falling$-$head permeability test in performed on a light$-$gray gravelly sanc $($SW$),$ and the following data is obtained:

$a=6\mathrm{.}25c{m}^2$

$A=10\mathrm{.}73c{m}^2$

$L=16\mathrm{.}28cm$

$h_1=160\mathrm{.}2cm$

$h_2=80\mathrm{.}1cm$

$t=90s$ for the head $to$ fall from $h_{1}to{h}_2$

Water temperature $=20C$

Required:

Compute the coefficient of permeability in centimeters per second.