Roasted copper ore containing the copper as CuSO₄ is to be extracted in a counter-current stage extractor. Each hour a charge consisting of 10 tons of inert solids 1.2 tons of copper sulfate, and 0.5 ton of water is to be treated. The strong solution produced is to consist of 90 percent H₂O and 10 percent CuSO₄ by weight. The recovery of CuSO₄ is to be 98 percent of that in the ore. Pure water is to be used as the fresh solvent. After each stage, 1 ton of inert solids retains 2 tons of water plus the copper sulfate dissolved in that water. Equilibrium is attained in each stage. How many stages are required?