| 1. A box contains 10 balls of the same size, indistinguishable to the touch: four blue, two white, three black and one
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| red.
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| 1.1. The ten balls will be removed, successively and at random, from the box and placed on a table, aligned by the
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| order in which they are removed.
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| Find the probability that the blue balls all stick together.
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| Express the result as an irreducible fraction.
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| 1.2. Suppose now that some more blue balls are placed in the box.
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| Two balls are drawn at random from the box, one after the other, without replacing the first one before removing the
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| second.
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| Knowing that the probability of drawing balls of different colors is equal to 13/
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| 21 determine the number of balls
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| added blues.
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