The vapour pressures of A and B at 25$^\circ$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)