An urn contains r red balls and w white balls.

An urn contains r red balls and w white balls. A sample of n balls is drawn in order and without replacement. Let Xi be 1 if the ith draw is red and 0 otherwise, i =1, 2, . . . , n.
(a) Show that E(Xi) = E(X1), i = 2, 3, . . . , n.
(b) Use the corollary to Theorem 3.9.2 to show that the expected number of red balls is nr/(r + w).