7. Let Z and W be jointly discrete random variables with the following joint probability mass function: 2 20 1 2 0.15 0.30 3 0.45 0.10 This means that P(Z - 3 and W = 2) - 0.10, for example. Find all of the following. (a) (1 point for each) E(Z), E(W) I (b) (2 points for each) Var(Z), Var(W). (c) (4 points) Cov(Z, W)