The tables below give the probability distributions for averages of two biology classes taught by two different teachers at a university. If a student from each class is selected at random, what is the expected difference in their grades? Teacher X xn 50 60 70 80 90 pn 0.05 0.1 0.2 0.35 0.3 Teacher Y yn 50 60 70 80 90 pn 0.05 0.05 0.05 0.8 0.05