Vapour pressure of pure 'A' is 70 mm of Hg at 25`^@` C. It forms an ideal solution with 'B' in which mole fraction of A is 0.8. If the vapour pressure of the solution is 84 mm of Hg at 25`^@` C, the vapour pressure of pure 'B' at 25`^@` C is
56 mm
70 mm
140 mm
28 mm