Can you post more data/information?
I don't think your CRE problem should be as simple as that, because regardless which reactor comes first, as long as the conversion of both reactors are fixed at 0.4, the overall conversion remains the same. An optimisation problem would arise if one can vary the operating parameters of each reactor.
I believe your CRE problem is an optimisation one.
In general, the operating line of a CSTR can be said to be a point on the operating line of a PFR.
You may consider an interstage cooler to furthur optimise the conversion.