A porous solid is dried in a batch dryer under constant drying conditions. Seven hours are required to reduce the moisture content from 35 to 10 percent. The critical moisture content was found to be 20 percent and the equilibrium moisture 4 percent. All moisture contents are on the dry basis. Assuming that the rate of drying during the falling-rate period is proportional to the free-moisture content, how long should it take to dry a sample of the same solid from 35 to 5 percent under the same drying conditions?