At 1 atmospheric pressure the boiling point of mixture is 80∘C.
At boiling point the vapour pressure of mixture, PT=1 atmosphere =760‌mm‌Hg.
Using the relation, PT=PA∘XA+PB∘XB, we get PT=520XA+1000(1−XA) {PA∘=520‌mm‌Hg,. PB∘=1000‌mm‌H‌g,XA+XB=1} or ‌‌760=520XA+1000−1000XA or 480XA=240 or ‌‌XA=‌