If we have 5 red, 4 white, 3 black and 2 yellow marbles, how many different 2 marble combinations are there? This is a really basic combinatorics question, yet I'm blanking. What is the general formula for that? The Manhattan GMAT book was not particularly thorough with this. Thanks all.

Hello The combination formula goes nCr where it is equal to n!/r!(n-r)! If you subsitute the values of picking r itmes from a set of n that is 2 marbles from a set of 4 you will get 6 as the result. Let me know what are the answer choices and if this is the correct solution. Thanks

But why are we paying no attention to the fact that there are 4 different colors?

Woozler wrote:If we have 5 red, 4 white, 3 black and 2 yellow marbles, how many different 2 marble combinations are there? The number of "different 2 marble combinations" is = 4C2 = 6 Why is so? Because a red-red pair is same as another red-red pair. Also a red-black pair is same as another red-black pair. Only way we can get "different 2 marble combination" if we consider each of them of different color. Therefore the problem reduces to: In how many ways 2 colors can be selected out of 4?

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