The balls are the stars. There are 6 of them and they are all the same.
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Now I will assume there can be zero balls in one or even two of the containers.
I have the containes in a row. Doesn't matter what order. Say red, blue green.
The bars represent dividers between the boxes. there will be 2 dividers
the red cont gets the balls on the left, the blue gets the middle balls and the green gets the remaining balls
These are the possible position of the bars (remembering that any 2 of them will be chosen.
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So there are 14 bars.
How many ways can 2 of them be chosen.
14C2 = 91 ways
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Of course if each container must have at least one ball then the problem is a bit different.
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The number of ways would be 5C2 = 10 ways
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I think that is right.