$(20\%)$ The excess Gibbs energy for a binary system consisting of two liquids is

given as $\frac{G^E}{RT}=A*x_i{x}_2,$ where $A$ is a function of temperature only and $x_i$ is the

mole fraction of the component i$.$ If this binary system always phase$-$separates

into two coexisting phases during $0\mathrm{.}3x_{\text{I}}0\mathrm{.}7,$ please determine the value of $A.$