What does E(X-Y) represent in a joint pdf The situation is that X and Y are proportions of correct answers on science and humanities exams respectively. Their joint pdf is
f(x,y) = .4(2x+3y) for X & Y = [0,1]
My homework asks for E(Y-X), which was easy enough, but what I don't understand is how would I interpret expected values like this? Would E(Y-X) simply be: the average proportion of correct answers on humanities minus average proportion of correct answer on science
And would such a simple explanation also be used to explain the significance of scenarios such as E(XY)?

The interpretation is: the average of the difference between the proportion of correct answers in science and the proportion of correct answers in humanities
In general if $g = g(X, Y)$ then $\mathbb{E}[g(X,Y)]$ means the average of $g(X, Y)$
