7. In washing clothes, 10kg surfactant solution was retained by 100kg completely exhausted dry clothes. Pure water is used to remove the surfactant from the tissue by leaching. It is known that the retained liquid on the clothes (0.1kg retained liquid 1kg dry clothes) is independent of the surfactant concentration. (a). By the method of cross-contact stage and 100kg water is allowable for the leaching, what is the recovery ratio after single-stage leaching? (5%) (b). As in problem (a), only 100kg water is allowable, how can you operate by crosscontact to remove 95% surfactant from the clothes? ( 4% ) (c). By using continuous counter-current contact leaching, and 30kg water is used as the solvent. What is the number of ideal stages required to remove 99% surfactant? (8%)