At 300 K vapour pressure of a pure liquid, ' A ' is 70 mm Hg . It forms an ideal solution with another liquid ' B '. The mole fraction of B in the solution is 0.2 and total vapour pressure of solution is 84 mm Hg at same temperature. What is the vapour pressure (in mm) of pure liquid B at 300 K ? [AP EAPCET 21 May 2025 S2]