Reversing Eq. 6 in Definitions and Mass Balance and dividing by R

Xa (1 + R)/R = Xr

(1a)

Subtracting Xa from both sides

(2a)

gives further

(3a)

Finally

Xa = R (Xr - Xa)

(4a)

Multiplication by OFR (symbol v ) yields:

vXa = vR(Xr - Xa)

(5a)

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