I need help with how to figure out the size of the single CSTR that would do the same job as the current setup of four series CSTR.

This class uses Fogler's Elements of Chemical Reaction Engineering.

Q #3. Reactant A ( AR, CAo = 125 mol/m?) passes in steady flow through four equal- sized CSTR's in series (Ttotal = 2 min). When steady state is achieved, the concentration of A is found to be 18, 5, 2, and 1 mol/m in the four reactors. For this reaction, what would be the space time in a single PFR to reduce the feed concentration from 125 to 1 mol/m?? What size of single CSTR would do the same job as the present four CSTRs