排列組合

We are given a red box,a blue box,and a green box.We also have 10 red balls,10 blueballs,and 10 green balls.Balls of the same color are considered identical.
Consider the following constraints:
(1) No box contains a ball that has the same color as the box
(2) No box is empty

請問這題怎麼解比較好呢?


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