A wet solid is dried from an initial moisture content of 0.5 kg water/kg dry solid to a final moisture content of 0.1 kg water/kg dry solid. The equilibrium moisture content is 0.05 kg water/kg dry solid. If the drying takes place entirely under falling-rate period and the drying rate is proportional to the free moisture content, what is the ratio of drying time to reduce moisture from 0.5 to 0.2 compared to 0.2 to 0.1.