The vapour pressures of $A$ and $B$ at $25^{\circ} \mathrm{C}$ are 90 mm Hg and 15 mm Hg respectively. If $A$ and $B$ are mixed such that the mole fraction of $A$ in the mixture is 0.6 , then the mole fraction of $B$ in the vapour phase is $x \times 10^{-1}$. The value of $x$ is $\_\_\_\_$ . (Nearest integer)