At $363 \mathrm{K}$, the vapour pressure of $\mathrm{A}$ is $21 \mathrm{kPa}$ and that of $\mathrm{B}$ is $18\mathrm{kPa}$. One mole of $\mathrm{A}$ and $2$ moles of $\mathrm{B}$ are mixed. Assuming that this solution is ideal, the vapour pressure of the mixture is___________ $\mathrm{kPa}$. $($Round of to the Nearest Integer$)$.