The vapour pressures of two volatile liquids A and B at 25 ° C are...

Since,

${y}_{A} = \dfrac{p{°}_{A} \times {x}_{A}}{p{°}_{A} \times {x}_{A} + p{°}_{B} \times {x}_{B}}$

and ${x}_{A} = 0 . 3$ and ${x}_{B} = 0 . 7$

${y}_{A} = \dfrac{50 \times 0 . 3}{50 \times 0 . 3 + 100 \times 0 . 7}$

${y}_{A} = \dfrac{15}{85}$.

$\dfrac{{Y}_{B}}{1 - {y}_{B}} = {P}_{B}^{0}{P}_{A}^{0}\left[\dfrac{{X}_{B}}{1 - {X}_{B}}\right]$

$⇒ \dfrac{{y}_{B}}{1 - {y}_{B}} = \dfrac{100}{50}\left[\dfrac{0 . 7}{0 . 3}\right] = \dfrac{14}{3}$

$⇒ {y}_{B} = \dfrac{14}{17}$