15. During the batch drying test of a wet slab of material 0.35 m² and 7 mm thick, the falling rate N was expressed as 0.95 (X-0.01) where N, is the drying rate in kg/m²s and X is the moisture content in kg moisture/kg dry solid. The constant drying rate was 0.38 kg/m²s and slab was dried from one side only with the edges sealed. Density of the dry material is 1200 kg/m³. It is desired to reduce the moisture content from 35% to 5% on wet basis. What is the time needed for drying? (Ans: 22.63 s)