$12\mathrm{.}10-3.$ A treated ore containing inert solid gangue and copper sulfate is to be leached in a countercurrent multistage extractor using pure water to leach the $CuS{O}_4.$ The solid charge rate per hour consists of $10000kg$ of inert gangue $($B$),1200kg$ of $CuS{O}_4($solute $A),$ and $400kg$ of water $($C$).$ The exit wash solution is to contain $92wt\%$ water and $8wt\%CuS{O}_4.$ A total of $95\%$ of the $CuS{O}_4$ in the inlet ore is to be recovered. The underflow is constant at $N=0\mathrm{.}5kg$ inert gangue solid $\frac{?}{kg}$ aqueous solution. Calculate the number of stages required.