The vapour pressure of two liquids P and Q are 80 and 60 torr...

By Raoult's Law $\mathrm{P}_{\mathrm{T}}=\mathrm{P}_{\mathrm{P}}^{\circ} \mathrm{X}_{\mathrm{P}}+\mathrm{P}_{\mathrm{Q}}^{\circ} \mathrm{X}_{\mathrm{Q}}$ $$\begin{aligned} & \mathrm{P}_{\mathrm{P}}^{\circ}=80 \text{ torr } \\ & \mathrm{X}_{\mathrm{P}}=\frac{3}{5} \\ & \mathrm{P}_{\mathrm{Q}}^{\circ}=60 \text{ torr } \end{aligned}$$ $$\begin{aligned} \mathrm{X}_{\mathrm{Q}} & =\frac{2}{5} \\ \mathrm{P}_{\mathrm{T}} & =80 \times \frac{3}{5}+60 \times \frac{2}{5} \\ & =48+24 \\ \mathrm{P}_{\mathrm{T}} & =72 \text{ torr. } \end{aligned}$$ Related Theory Solutes which dissociate or associate in a particular solution will not obey Raoult's law. Raoult's law applies to solutions containing non-volatile solute only. Raoult's law cannot be applied to concentrated solutions.