$(13$ points$)$Dry a wet material under constant drying condition. The drying

rate curve in the fall$-$rate period is a straight line. The drying rate $R_c$ of the

constant$-$rate period is $1\mathrm{.}5kg(m^2*h),$ the critical water content $x_4$ is $0\mathrm{.}2,$ and

the equilibrium water content $x^t$ is $0\mathrm{.}05.$ The dry area for the wet material

containing $40kg$ bone$-$dry content is $1m^2.$ The weight of the wet material is

$2000kg.$ Calculate the required drying time to reduce water content from

$x_i=0\mathrm{.}35$ to $xg=0\mathrm{.}1.$