the red function curve is the 1/r - curve for a 'simple' reaction of first order, - wanted conversion: 0.9
First alternative:
first reactor: CSTR, second reactor FTR
REM: the area under the rightangle is a measure for the space time in the CSTR, the area under the function curve is the measure for the space time in the TFR.
Second alternative:
first reactor: TFR , second reactor: CSTR

REM: area under the first part of the function curve is a measure for the spacetime of the TFR, area of the 'big' rightangle is a measure for the spacetime of a CSTR.
Don't you think that this visualization needs no further comment !!!