Penicillin is extracted from a fermentation broth using isoamylacetate as the organic solvent in a continuous countercurrent cascade extraction unit. The flow rates of organic (S) and aqueous (F) phases are S=10l/min and F=100 1/min, respectively. The distribution coefficient of penicillin between organic and aqueous phases at pH=3 is K=y/x=50. If the penicillin concentration in the feed stream is 20g/l, determine the number of stages required to reduce the penicillin concentration to 0.1g/l in the effluent of the extraction unit