An urn contains 30 balls, of which 10 are red and 8 are blue. From this urn, 12 balls are randomly withdrawn. Let X denote the number of red and Y the number of blue balls that are withdrawn. Find Cov(X, Y)
(a) By defining appropriate indicator (that is, Bernoulli) random variables
(b) By conditioning (on either X or Y) to determine E[XY].