Reversing Eq. 6 in Definitions and Mass Balance and dividing by R
X_{a} (1 + R)/R = X_{r} 
(1a) 
Subtracting X_{a} from both sides

(2a) 
gives further

(3a) 
Finally
X_{a} = R (X_{r}  X_{a}) 
(4a) 
Multiplication by OFR (symbol v ) yields:
vX_{a} = vR(X_{r}  X_{a}) 
(5a) 
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