At 300   K , the vapour pressure of a solution containing 1 mole of n -hexane and 3 moles of n -heptane…

${\mathrm{P}}_{\mathrm{total}}={{\mathrm{P}}^{\circ }}_{\mathrm{hexane}}.{\mathrm{X}}_{\mathrm{hexane}}+{{\mathrm{P}}^{\circ }}_{\mathrm{heptane}}.{\mathrm{X}}_{\mathrm{heptane}}$

$550=\left[{{\mathrm{P}}^{\circ }}_{\mathrm{hexane}}\right]\times \frac{1}{4}+\left[{{\mathrm{P}}^{\circ }}_{\mathrm{heptane}}\right]\times \frac{3}{4}$     ..$\left(\mathrm{i}\right)$

After mixing $1$ mole n-heptane

$560=\left[{{\mathrm{P}}^{\circ }}_{\mathrm{hexane}}\right]\times \frac{1}{5}+\left[{{\mathrm{P}}^{\circ }}_{\mathrm{heptane}}\right]\times \frac{4}{5}$     .....$\left(\mathrm{ii}\right)$

On solving eq. (i) and (ii)

${{\mathrm{P}}^{\circ }}_{\mathrm{heptane}}=600\mathrm{mm}$ of $\mathrm{Hg}$