Two liquids \( \mathrm{A} \) and \( \mathrm{B} \) form an ideal solution at temperature \( T \, \text{K} \). At \( T \, \text{K} \), the vapour pressures of pure \( \mathrm{A} \) and \( \mathrm{B} \) are \( 55 \) and \( 15 \, \text{kN m}^{-2} \) respectively. What is the mole fraction of \( \mathrm{A} \) in solution of \( \mathrm{A} \) and \( \mathrm{B} \) in equilibrium with a vapour in which the mole fraction of \( \mathrm{A} \) is \( 0.8 \)?