The composition of vapour over a binary ideal solution is determined by the composition of the liquid. If `X_(A)` and `Y_(A)` are the mole fractions of `A` in the liquid and vapour, respectively, find the value of `X_(A)` for which `Y_(A)-X_(A)` has a maximum. What is the value of the pressure at this composition in terms of `P_(A)^(@)` and `P_(B)^(@)`.