$6\mathrm{.}4.$ Reactant $)=(26\frac{\text{m}\text{o}\text{l}}{m^3}$ passes in steady flow through four equal$-$size mixed flow reactors in series $)=(2(min).$ When steady state is achieved the concentration of $A$ is found to be $11,5,2,1mo\frac{l}{m^3}$ in the four units. For this reaction, what must be ${}_{\text{p}\text{l}\text{u}\text{g}}$ so as to reduce $C_A$ from $C_{A0}=26$ to $C_{Af}=1mo\frac{l}{m^3}?$