Q.

A bag contains 10 white and 3 black balls. Balls are drawn without replacement till all the black balls are drawn What is the probability that this procedure will come to an end on the seventh draw ?

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a

1/281

b

1/211

c

15/286

d

1/286

answer is A.

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Detailed Solution

Out of the 3 black balls 2 should have been drawn in the first six draws so that when the third appears in the seventh draw the procedure ends. In the first six draws 4 white balls and 2 black balls should  have been drawn so that the required probability
=6C41013912811710392817=15286

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