The elementary gas phase reaction

$A+BC+D$

takes place isobarically and isothermally in a PFR where $50\%$ conversion is achieved. The feed is equimolar in A and $B.$ It is proposed to put a CSTR of equal volume upstream, i$.$e$..$

of the PFR$.$ What will be the intermediate and exit conversions $x_1$ and $x_2$ respectively? The entering flow rates and all other variables remain the same as that for the single PFR$.$