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# The cards in a deck are numbered consecutively from 1 to 20.

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Manager
Status: GMAT Preperation
Joined: 04 Feb 2010
Posts: 103
Concentration: Social Entrepreneurship, Social Entrepreneurship
GPA: 3
WE: Consulting (Insurance)
Followers: 2

Kudos [?]: 110 [0], given: 15

The cards in a deck are numbered consecutively from 1 to 20. [#permalink]

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16 Oct 2010, 05:47
00:00

Difficulty:

55% (hard)

Question Stats:

36% (01:34) correct 64% (00:39) wrong based on 25 sessions

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The cards in a deck are numbered consecutively from 1 to 20. If some cards are red and some
cards are black, what is the probability of drawing a red card?
(1) The odd-numbered cards are all red.
(2) The even-numbered cards are all black.
[Reveal] Spoiler: OA
Intern
Status: Applying
Joined: 14 Oct 2009
Posts: 31
Location: California
Schools: Cornell AMBA, Kellogg, Oxford, Cambridge
Followers: 0

Kudos [?]: 48 [0], given: 4

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16 Oct 2010, 14:55
I think the catch here is that "some cards are red and some cards are black". May be some cards are of other colors too. This would prevent us from assuming that each of the given statements are sufficient and requires both together to determine the probability.
Manager
Joined: 19 Aug 2010
Posts: 77
Followers: 3

Kudos [?]: 24 [0], given: 2

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16 Oct 2010, 15:21
Fremontian I agree with you.

According to (1) there is still the possibility that some of the even-numbered cards could be also red and if I don't consider that I will falsely underestimate the probability of drawing a red card, so insufficient.

Statement (2) ensures me that the rest of the cards are black,
hence I can exactly estimate the probability if I take both statements together.

Therefore C
Joined: 19 Feb 2010
Posts: 401
Followers: 22

Kudos [?]: 160 [0], given: 76

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17 Oct 2010, 03:28
Haha, what a trick. It's good to be aware of this kind of questions. If we wanted the answer D, then it should read something like that:

(1) The odd-numbered cards "only" are all red.
(2) The even-numbered cards "only" are all black.
Re: deceptive problem   [#permalink] 17 Oct 2010, 03:28
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