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Karen has exactly 6 quarters, 5 dimes, and 10 nickels in her

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Manager
Joined: 21 May 2011
Posts: 241
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Karen has exactly 6 quarters, 5 dimes, and 10 nickels in her [#permalink]  07 Aug 2011, 13:10
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Difficulty:

75% (hard)

Question Stats:

30% (02:43) correct 70% (03:00) wrong based on 10 sessions
Karen has exactly 6 quarters, 5 dimes, and 10 nickels in her pocket. If she withdraws a number of these coins at random, how many coins would she have to withdraw to ensure that she has at least a 50 percent chance of withdrawing at least one quarter?

A. 1
B. 2
C. 5
D. 6
E. 7
[Reveal] Spoiler: OA
Manager
Status: Writing my first GMAT on MArch 28/2012
Joined: 13 Sep 2010
Posts: 124
Schools: Schulich, Rotman '15
GMAT Date: 03-28-2012
GPA: 2.8
WE: Corporate Finance (Other)
Followers: 1

Kudos [?]: 26 [0], given: 9

Re: PS - 700 level - quarter [#permalink]  07 Aug 2011, 13:20
Hey this question was posted already.

Basically there are 21 coins overall, now the odds of picking a quarter are 6/21= .286. Then it is 6/20 = .3.
When you treat each event individually you get those percentages, Adding these two probabilities together are ~.59

So the answer is B or 2.

Here is the link of the question previously posted.
probability-confusing-72435.html
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Joined: 24 Mar 2011
Posts: 459
Location: Texas
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Kudos [?]: 95 [0], given: 20

Re: PS - 700 level - quarter [#permalink]  07 Aug 2011, 13:23
bschool83 wrote:
Karen has exactly 6 quarters, 5 dimes, and 10 nickels in her pocket. If she withdraws a number of these coins at random, how many coins would she have to withdraw to ensure that she has at least a 50 percent chance of withdrawing at least one quarter?

1
2
5
6
7

i will go with choice 2nd here..
suppose that she does not with draw quarter in 1st.. then probability of dime or nickel = 15/21 = 5/7 ~ 0.72
and probability of withdrawing quarter in 1st is = 0.28

if the 1st coin is dime or nickel then the probability of withdrawing quarter 2nd time is 6/20 = 3/10 = 0.3

together - 0.28 + 0.3 = 0.58 which is greater than 0.5 or 50% chances of withdrawing quarter..
so she should withdraw quarter in 1 of the 2 chances..
Re: PS - 700 level - quarter   [#permalink] 07 Aug 2011, 13:23
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