At $27^{\circ} \mathrm{C}$, two liquids A and B form an ideal solution with mole fractions 0.67 and 0.33 respectively. If the vapour pressure of pure $A$ and $B$ at $27^{\circ} \mathrm{C}$ are $300 \mathrm{~mm}$ and $450 \mathrm{~mm}$ respectively, the total vapour pressure of the solution in $\mathrm{mm}$ is: