# math

an urn contains 2 white and 2 black balls. a ball is taken at random. if the ball is white then it is not replaced otherwise it is replaced along with another ball of the same colour. find the probability that the third ball drawn is black.

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1. and your answer is? / let me know what you think first.

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2. list the possibilities:
P(WWW) = 2/4 * 1/3 * 0/2
P(BWW) = 2/4 * 2/5 * 1/4
...
P(BBB) = 2/4 * 3/5 * 4/6
then add up all the ones where the 3rd ball is black.

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