From Raoult's law, pT=pA∘χA+pB∘χB..........(i) where, pT= total pressure of solution pB∘,pA∘= vapour pressure of pure liquid B and A respectively. χA,χB= mole fraction of A and B in solution phase. Given, pA∘=21kPa,pB∘=18kPa nA=1,nB=2 χA=‌