An urn haves 30 balls, of which 10 are red and 8 are blue. From this urn, 12 balls are randomly withdrawn. Let X denote number of red and Y the number of blue balls which are withdrawn.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 find out E[XY]