Liquids A and B form ideal solution for all compositions of A and B...

${P}_{\text{Total}} = {P}_{A}^{o}{X}_{A} + {P}_{B}^{o}{X}_{B}$

$( i ) 0 .3 = ( {P}_{A}^{o} ) \dfrac{1}{4} + ( {P}_{B}^{o} ) \dfrac{3}{4}$

${P}_{A}^{∘} + {P}_{B}^{∘} = 1 . 2$

$( ii ) 0 .4 = ( {P}_{A}^{o} ) \dfrac{1}{2} + ( {P}_{B}^{o} ) \dfrac{1}{2}$

${P}_{A}^{∘} + {P}_{B}^{∘} = 0 . 8$

From equation (1) and equation (2)

${P}_{B}^{o} = 0 .2 bar$ and ${P}_{A}^{o} = 0 .6 bar$