Vapour pressure of pure 'A' is 70 mm of Hg at $25^{\circ}C$. It forms an ideal solution with 'B' in which mole fraction of A is 0.8. If the vapour pressure of the solution is 84 mm of Hg at $25^{\circ}C$, the vapour pressure of pure 'B' at $25^{\circ}C$ is