SOLUTION: if a box contains 4 red balls, 5 blue balls, and 6 green balls, suppose that they are selected one by one until a green ball is found. what is the probability that it is necessary

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Question 563087: if a box contains 4 red balls, 5 blue balls, and 6 green balls, suppose that they are selected one by one until a green ball is found. what is the probability that it is necessary to examine at least 6 balls?
what i've tried. obviously, there are 15 balls total, and by the combination formula, 15 nCr 6 (in TI-83 language, in English, 15 choosing 6) there are 5005 possible combinations when you choose 6 from a total of 15. I am just stumped on how to get the numerator to divide by 5005.
Any insight would be greatly appreciated. Thank you.
Answer by scott8148(6628)   (Show Source): You can put this solution on YOUR website!
another way to look at the question ___ what is the probability of drawing 5 consecutive non-green balls?

ways to draw 5 of 15 ___ 15C5

ways to draw 5 of 9 ___ 9C5

(9C5) / (15C5)
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