${P_B}\, = \,P_B^o\, \times {x_B}\, = $  total pressure $\times y_B$

where $x$ and $y$ represents mole fraction in liquid and vapour phase repsectively.

$\frac{{P_B^o{x_B}}}{{P_A^o{x_A}}}\, = \,\frac{{{y_B}}}{{{y_A}}};$

$\frac{{P_B^o(1 - \,{x_A})}}{{P_A^o\,{x_A}}}\, = \,\frac{{1 - \,{y_A}}}{{{y_A}}}$

on putting values $\frac{{119\,(1 - \,0.50)}}{{37\, \times \,0.50}}\, = \,\frac{{1 - \,{y_A}}}{{{y_A}}}$

on solving $y_A\,=\,0.237$