Reactant A (AR, C A0 = 26 mol/m 3 ) passes in steady flow through four equal-size

Question:

Reactant A (A→R, C_A0 = 26 mol/m³) passes in steady flow through four equal-size mixed flow reactors in series (τ_total = 2 min). When steady state is achieved the concentration of A is found to be 11, 5, 2, 1 mol/m³ in the four units. For this reaction, what must be π_plug so as to reduce C_A from C_A0 = 26 to C_Af = 1 mol/m³?