In an extraction system, the solute contained in a mineral is extracted in multiple countercurrent stages. 0.5 ton of inert solid, 0.6 ton of solute and 0.25 ton of water (with the mineral) enter the extraction system per hour. It is intended to recover 90% of the solute in the extract, with a concentration of 0.05 kg of solute/kg of solution. The retained solution is 2 kg/l kg of inert. Determine the necessary number of theoretical stages.