Zn is to be recovered from an ore containing ZnS. The ore is first roasted with oxygen to produce ZnO, which is leached with aqueous H₂SO₄ to produce water-soluble ZnSO₄ and an insoluble, worthless residue called gangue. The decanted sludge of 20,000 kg/h contains 5 wt% water, 10 wt% ZnSO₄, and the balance as gangue. This sludge is to be washed with water in a continuous, countercurrent washing system to produce an extract, called a strong solution, of 10 wt% ZnSO₄ in water, with a 98% recovery of ZnSO₄. Assume that underflow from each washing stage contains, by weight, two parts of water (sulfate-free basis) per part of gangue. Determine the stages required.