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There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random

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There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random [#permalink]

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New post 02 Oct 2007, 13:34
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There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks randomly distributed in a certain field. If 2 rocks are to be chosen at random and without replacement, what is the probability that both rocks will be slate rocks?

A) 1/3
B) 7/22
C) 1/9
D) 7/66
E) 2/45
[Reveal] Spoiler: OA
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Re: There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random [#permalink]

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New post 02 Oct 2007, 13:57
Total No of Rocks = 45

Probability of choosing 1st Slate Rock = 15/45
Probability of choosing 2nd Slate Rock = 14/44 (Without replacement)

So combined probability = 15/45*14/44 = 7/66

So, D.

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Re: There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random [#permalink]

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New post 04 Oct 2007, 00:39
avenger wrote:
Q34)
There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks randomly distributed in a certain field. If 2 rocks are to be chosen at random and without replacement, what is the probability that both rocks will be slate rocks?

(a) 1/3
(b) 7/22
(c) 1/9
(d) 7/66
(e) 2/45



15/45 * 14/44 ---> 7/66.

Just add up the total rocks. Then u calculate first prob, then u must subtract the one slate rock from both numerator and denominator.

Ans D.
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Re: There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random [#permalink]

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New post 04 Oct 2007, 15:53
OA is D.

Thanks guys. At this point I was just blindly guessing due to the time constraints... if i had the time I would have gotten it right...aaarrrgghhh! :-)

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Re: There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random [#permalink]

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Re: There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random [#permalink]

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New post 18 Apr 2016, 09:21
Hello from the GMAT Club BumpBot!

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There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random [#permalink]

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New post 18 Apr 2016, 11:23
Total no of rocks is 45

Probability of selecting 2 slate rocks ( Of 15 slate rocks ) out of 45 total Rocks is

15C2/45C2

=> 105 / 990

=> 21/198

=> 7/66

Hence answer will be (D) 7/66 :-D :lol:
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There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random   [#permalink] 18 Apr 2016, 11:23
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There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks random

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