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If a bowl contains only white and black balls, what is the

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Director
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If a bowl contains only white and black balls, what is the [#permalink] New post 16 Mar 2011, 16:45
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A
B
C
D
E

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82% (01:32) correct 18% (00:37) wrong based on 39 sessions
If a bowl contains only white and black balls, what is the probability of extracting a white ball from the bowl?

(1) There are twice as many white balls as black balls in the bowl
(2) If two balls are extracted from the bowl, the probability that both of them will be black is 0.

Hey Guys
Please let me know if from S2 its ok to assume -

a there are zero black balls in the bowl - I am thinking about this word "only" a bowl contains only white and black balls. I believe this assumption is correct since its even more restrictive.

b there is one black ball in the bowl. The OE assumes this.
[Reveal] Spoiler: OA
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Re: probability [#permalink] New post 16 Mar 2011, 18:59
The answer is A.

From(1) -> W/B+W = W/(W/2 + W) = 2/3, so sufficient.

From (2), we can only infer that # of Black balls <= 1, but no info about White balls

Yes, we can assume from S2 that there are 0 or 1 black ball. because the probability is 0 can imply three things :

The probablity = B1/N * B2/(N-1)

B1 = # of black balls in container during first draw

B2 = # of black balls during second draw

N = Total # of balls

Because the probability of this even is zero, so:

1) First drawn ball is black but the second is not
2) Second drawn ball is black but the first is not
3) Both the balls are not black.
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Director
Director
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Status: Matriculating
Affiliations: Chicago Booth Class of 2015
Joined: 03 Feb 2011
Posts: 931
Followers: 11

Kudos [?]: 196 [0], given: 123

Reviews Badge
Re: probability [#permalink] New post 16 Mar 2011, 19:30
Thanks man!

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Re: probability   [#permalink] 16 Mar 2011, 19:30
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