I completely forget how to do this, any help would be greatly appreciated.
Alex has a bag containing 5 yellow, 6 blue, and 7 purple marbles. Find the probability of him choosing, without replacement, 2 purples and a blue in any order in 4 picks.
(^If it were 3 picks, I'd have no problem, but I forget how to do this type of problem...)
I recommend to divide the problem into cases.
Case 1: 2 purble, 1 blue, 1 yellow.
Case 2: 2 purble, 1 blue , 1 blue = 2 purble , 2 blue.
Case 3: 2 purble, 1 blue, 1 purble = 3 purble , 1 blue.
Now, you find the probability for each one, which is much easier. And them you add them up.