Mole fraction of the benzene in the vapour phase which is in equilibrium with a solution of benzene $(p^0=120torr)$ and toluene$(p^0=80torr)$ having 2.0mole of each is

Question

$\begin{array}{1 1}(a)\;0.25\\(b)\;0.50\\(c)\;0.60\\(d)\;0.40\end{array}$