5 red, 7 blue, 8 green.  Two drawn consecutively and without replacement. Probability of at least one being blue?
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Chance of first marble not being blue is 13/20, and the chance of the second marble not being blue is 12/19, because a green or red marble has been drawn leaving 19 marbles, 7 of which are blue. So the chances of not drawing a blue marble in two trials is 13/20*12/19=156/380=39/95. Taking this away from 1 is 56/95 which is the chance of at least one blue marble being picked.

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