Wood that contains 20 wt% water is dried with hot air at a lumber mill. The hot air entering the drier has a water content of 2 wt% (at a lower content the wood would warp or split because it dries it too quickly). The wood is dried to 5 wt% water. The air leaving the drier contains 4 wt% water. How much air (kg/h) is required to 100 kg/hr of the wet wood?