The soluble solids are extracted from coffee with water. this extraction is performed in a multistage system that operates counter currently, where 3000 kg/h of roasted and ground coffee contains 24% in weight of soluble solids, while the moisture content is negligible. five percent of the soluble solids contained in the food appear in the final underflow stream, while the overflow stream contains 35% in weight of such solids. from the previous laboratory experiments, it was obtained that the amount of solution retained by inert solids is 1.6 kg solution per kg inert solids. calculate the number of stages needed if the efficiency of the system is 80%.