Liquids $A$ and $B$ form an ideal solution. At a certain temperature the total vapour pressure of a mixture of $A$ and $B$ is $400mm$. The mode fraction of $A$ in the liquid mixture in equilibrium with the vapour phase is $0.4$. If the vapour pressure ratio $\left( { P }{ A }^{ o }/{ P }{ B }^{ o } \right) $ for the pure liquids at this temperature is $1/6$, what is the partial pressure of $A$ in the vapour phase?