Oil is to be extracted from a meal with the use of benzene as solvent. The unit is to treat $1000$ kg of meal containing $200$ kg oil $($and the rest solid$)$ per hour. The wash solution contains $10$ kg oil dissolved in $650$ kg benzene.The discharged solids are to contain $60$ kg unextracted oil. Each kg of solid retains $0\mathrm{.}50$ kg solution. Use the analytical solution to determine the number of required stages.