Given, $P^o_A\,=\,74.7\,torr$, $P^o_B\,=\,22.3\,torr$

$n_{benzene}\,=\,1.5\,mol$, $n_{toluene}\,=\,3.5\,mol$

$n_{solution}\,=\,1.5+3.5=5\,mol$

$X_A\,=\,\frac {n_{benzene}}{n_{solution}}\,=\,\frac {1.5}{5}$

Total $V.P.$ of solution

$ = \left( {\frac{{1.5}}{5}\, \times \,74.7\, + \,\frac{{3.5}}{5}\, \times \,22.3} \right)\,torr$

$=\,(15.6+22.4)\,torr\,\,=\,38\,torr$

Mole fraction of benzene in vapour form

$=\,\frac {22.4}{38}\,=\,0.589$