A consolidation test on a clay specimen $($drained on both sides$),$ the clay layer has a $-$thickness of $25$ mm

$-$ loaded stress level of $400$ kN$/$m$^2.$

$-$ The total consolidation settlement under this stress level is $4$ mm$.$

What is the time it took for the first $2$ mm consolidation settlement to occur? $($The coefficient of consolidation cv is $1\mathrm{.}83$x$10-7$ m$^2/$s$).$

Please show all your steps