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
The target is to obtain saturated pressure of water at 200 ºC using PHI-PHI (fugacity coefficients) approach,so it means it's a VLE of water only,so we write equilibrium condition:
$f_i(l)=f_i(v)$ (*)
which $f_i(l)/P=PHI(l)$ and same for $PHI(v)$ (note that $Y_i=X_i=1$ ) ,substituting into (*) ...