Liquids A and B form an ideal solution. At ${50}^0\mathrm{C}$ , the total vapour pressure of a solution containing 1 mole of A and 2 moles of ‘B’ is 300 mm Hg. When 1 mole of ‘A’ is added to the solution the total vapour pressure increases to 400 mm Hg. Then vapour pressure of pure ‘B’ is ____mm  Hg