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Algebra

Blazing goIITian

Joined:	18 Feb 2007
Post:	1042

27 May 2008 15:21:32 IST

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302

probability DCE m-t

None

A bag contains 10 white & 3 black balls.Balls are drawn one by one without replacement till all the black balls are drawn. The probability that the procedure of drawing balls will come to an end at the seventh draw is

1) 105/286 2) 15/286

3) 181/286 4) none of these

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SUNDEEP ALLAMRAJU

Blazing goIITian

Joined: 28 Feb 2008

Posts: 1014

27 May 2008 15:40:55 IST

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Ans is 15/286.

The procedure will end at the 7th draw iff 7th draw is that of the 3rd black ball.That means,in the previous 6 draws,2Black and 4white are drawn.Hence,the probability is

P(E)=(3c₂.10c₄/13c₆)(1/7)=(105/286)(1/7)=15/286.

After 6th draw,there are 6W and 1B left,so,probability of drawing Black in 7th draw is (1/7).Hope it's clear to you.

Priyesh

Blazing goIITian

Joined: 18 Feb 2007

Posts: 1042

27 May 2008 16:20:31 IST

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ya ure absolutely correct !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Thanks

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