The vapour pressures of pure liquids A and B are 400 and 600mmHg...

Raoult’s law for ideal solution,

${P}_{Total} = {P}_{A} + {P}_{B}$

${P}_{Total} = {x}_{A} \times {P}_{A}^{∘} + {x}_{B} \times {P}_{B}^{∘}$

${P}_{Total} = \dfrac{1}{2} \times 400 + \dfrac{1}{2} \times 600 = 500mmofHg$

Dalton’s law,

${y}_{A} \times {P}_{Total} = {P}_{A}$

${y}_{A} = \dfrac{{x}_{A} \times {P}_{A}^{0}}{{P}_{Total}} = \dfrac{1}{2} \times \dfrac{400}{500} = 0.4$

${y}_{B} = 1 - 0.4 = 0.6$