Question 370137: In a strings of length 9 with elements being a, b or c. How many strings will contain at least 6 b’s? Answer by robertb(5830) (Show Source):
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The first term can be explained as follows: Pick any 6 places out of a possible 9. These places will be occupied by b's. The remaining three places can be filled by a or c (2 choices!), and can be done in ways, assuming repetition. Same explanation applies for the other terms.
