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# Q34) There are 15 slate rocks, 20 pumice rocks, and 10

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Manager
Joined: 30 Aug 2007
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Q34) There are 15 slate rocks, 20 pumice rocks, and 10 [#permalink]  02 Oct 2007, 12:34
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Question Stats:

100% (01:45) correct 0% (00:00) wrong based on 4 sessions
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
Director
Joined: 17 Sep 2005
Posts: 931
Followers: 2

Kudos [?]: 23 [0], given: 0

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.

- Brajesh
CEO
Joined: 29 Mar 2007
Posts: 2593
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Kudos [?]: 189 [0], given: 0

Re: GMAT Prep 2: PS Q34 [#permalink]  03 Oct 2007, 23:39
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.
Manager
Joined: 30 Aug 2007
Posts: 62
Followers: 1

Kudos [?]: 1 [0], given: 0

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!

M.
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