Hi; I'm having trouble matching up my combinations probability and binomial expansion probability I have 16 marbles 5 red, 11 blue and I want to know the probability of getting 2 red and 2 blue when four are picked.for the binomial I have (5/16 + 11/16)^4 expanding to 1(5/16)^4 + 4(5/16)^3(11/16) + 6(5/16)^2(11/16)^2which gives 6(5/16)^2(11/16)^2 = 0.2769 Now for the combination (5c2)(11c2)/(16c4) = 0.3022Don't understand why they have different answers?Thanks.