For an ideal solution obeying Raoult's Law, total pressure is
P=PAoxA+PBoxB.
Initially with 2 moles A and 3 moles B:
xA=0.4,xB=0.6, and P=320=0.4PAo+0.6PBo
After adding 1 mole each: 3 moles A and 4 moles B gives
xA=73,xB=74, and P=328.6=73PAo+74PBo
From the first equation:
3200=4PAo+6PBo
From the second:
2300.2=3PAo+4PBo
Solving: We find PBo=200 mm Hg and PAo=500 mm Hg.