Chemistry · Solutions and Colligative Properties

Two liquids A and B form an ideal solution. At 320 K, the vapour pressure of the solution, containing 3 mol of A and 1 mol of B is 500 mm Hg. At the same temperature, if 1 mol of A is further added to this solution, the vapour pressure of the solution increases by 20 mm Hg. What is the vapour pressure (in mm Hg) of B in the pure state?