At ${80}^{\circ }\mathrm{C}$, the vapour pressure of pure liquid $\text{ 'A'}$ is $520\mathrm{mm} \mathrm{Hg}$ and that of pure liquid $\text{'}\mathrm{B}\text{'}$ is $1000 \mathrm{mm} \mathrm{Hg}$. If a mixture of solution $\text{'}\mathrm{A}\text{'}$ and $\text{'}\mathrm{B}\text{'}$ boils at $80°\mathrm{C}$ and $1$ atm pressure, the amount of $\text{'}\mathrm{A}\text{'}$ in the mixture is $(1 \mathrm{atm} = 760 \mathrm{mrn} \mathrm{Hg})$