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An urn contains 9 white, 9 red, 9 blue and 9 green balls. 4 balls are drawn randomly without replacement and a fifth bonus ball is drawn from the same urn.

a) What is the probability of drawing 3 red balls and a non-red bonus ball?

b) What is the probability of drawing 2 black balls and 1 black bonus ball?

I managed to use a general formula for drawing 5 balls but I cannot modify it for the required solution. Please refer to the attached images for my progress.

Drwing 5 Balls straight

Drawing 4 + 1 balls

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  • $\begingroup$ Hint: look at the balls drawn as a random permutation, and work on that $\endgroup$ – Mickey Jun 23 '17 at 11:59
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    $\begingroup$ On a) apply: P(3 red and non-red bonus)=P(non-red bonus|3 red)P(3 red). Sortlike for b). $\endgroup$ – drhab Jun 23 '17 at 12:30

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