Use Bayes' theorem, let a denote the sudent from A and b the sudent from B:
P(A is sup| a fair & b poor)=P(a fair & b poor|A is sup) P(A is sup)/P(a fair & b poor)
...............=(10/30 15/30)(0.5)/[(10/30 15/30)0.5 + (10/30 5/30)0.5]
...............~=0.083333/[0.083333+0.027778]~=0.75
(Check the calculations please)
RonL