In a countercurrent extractor, NaOH is extracted from the CaCO3-NaOH mixture with pure water. The feed mixture contains 1000 kg/h CaCO3 (inert), 100 kg/h NaOH, and 50 kg/h H2O. The extract phase is 90% water and 10% NaOH by weight. In the case of constant downstream, 1 ton of solution (water + NaOH) per 1 ton of inert is retained. As a result of the extraction, 98% of the NaOH is extracted. How many ideal stages are needed, assuming each stage is in equilibrium?