Calculation was made for a single$-$stage steam$-$stripping process to remove a taint from cream. The conditions were that stage$-$contact desorption was to be used to remove a taint that was present at a concentration of $10$ ppm in the cream, by contact with a counter flow current of steam. Consider, now, the case of a rather more difficult taint to remove in which the equilibrium concentration of the taint in the steam is only $7\mathrm{.}5$ times as great as that in the cream.

If the relative flow rates of cream and steam are given in the ratio $1$:$0\mathrm{.}75,$ how many contact stages would be required to reduce the taint concentration in the cream to $0\mathrm{.}3$ ppm assuming $($a$)100\%$ stage efficiency and $($b$)70\%$ stage efficiency? The initial concentration of the taint is $10$ ppm$.$