A slab with a wet weight of

originally contains $40$ percent moisture $($wet basis$).$ The slab is $600$ by $900$ by

thick. The equilibrium$-$moisture content is $6$ percent of the total weight when in contact with air of

and $20$ percent humidity. The drying rate is given in the following table for contact with air of the above quality at a definite velocity. Drying is from one face. How long will it take to dry the slab to $18$ percent moisture content $($wet basis$)?$