The aqueous reversible reaction AB, r = kCA - KCB is to be carried out in a series of reactors with separation of unreacted B between stages. At each stage the reaction goes to 50% of the equilibrium composition. How many stages are required for 90% conversion of the initial A for k = 0.2 min-!, Keq - 1 if (a) all B is extracted between stages, (b) 50% of B is extracted between stages