please solve example 2-3

the solution is there but idk how he got v1 and v2. please show

Example 2-2 The feed to two CSTR's in series is 0.4mol/s. If 40% conversion is achieved in the first reactor what would be the volume of each reactor necessary to achieve 80% overall conversion of the entering species A? Solve example 2-2 for two PFR's in series as shown in the diagram Solution- PFR in Series Find V1 and V2 ? Vi=Xi1XirAFA0dX PFR in series are equivalent to one PFR with the same total volume as well as achieving same conversion! Read Example 2-6 in Fogler