A urn contains 90 marbles, of which there are 20 green, 20 black and 50 red marbles. Tom draw marbles without replacement until the 6th green marble. Let X = # of marbles drawn. Example : GGGBRBGGBRG with x = 11. a). Find the probability P(X = 20), 14 points) b). Find the expectation EX. [4 points c). Find the variance VarX.
