Penicillin is recovered from an aqueous stream $)=(1000\frac{kg}{m^3}$ in a solvent extractor mixer$-$settler unit using MIBK $({}_{\text{M}\text{I}\text{B}\text{K}}=797\mathrm{.}8k\frac{g}{m^3})$ as a solvent. For a feed of $25\frac{m^3}{h}$ containing $10k\frac{g}{m^3}$ of penicillin,

$($a$)$ Determine the volumetric flow rate of solvent required if the final penicillin concentration in the aqueous stream is to be reduced by $90\%.$

$($b$)$ If a second mixer settler is available determine the fresh solvent feed needed to reduce the aqueous penicillin stream from the first unit by a further $90\%.$

Note: the equilibrium relationship between the penicillin concentration in the organic $(k\frac{g}{m^3})$ and aqueous $(k\frac{g}{m^3})$ phases is$,$

$K=\frac{C_{\text{o}\text{r}\text{g}}}{C_{eq}}=20$

If instead of the cross flow configuration of $15(b)$ above a counter current flow arrangement was used, in which the exit solvent from the second stage acted as the solvent feed to the first stage, determine the overall solvent usage and compare this usage to that of $($a$)$ and $($b$)$ above.

I've solved Q$15,$ the answers to both parts are $11\mathrm{.}25$m$^3/$h$.$ I only need help with Q$16$