Benzoic acid is continuously extracted from toluene using wateras solvent in a cascade of stages in series. Each stage consist of: (a) A mixture where vigorous stirring of the contents takes place and (b) A settler where the mixture pumped from the mixer is allowed to settle into layers. The upper toluene layer and the lower water layer are removed separately. Develop a mathematical model to find the final fraction of benzoic acid extracted for the case of counter-current operations