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# John must select a marble from a bag containing red,green

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Director
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John must select a marble from a bag containing red,green [#permalink]  25 Jul 2009, 02:51
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John must select a marble from a bag containing red,green and blue marbles.If there are 20 marbles or fewer in the bag,is the probability that he will select a red marble greater than 1/4?
1.there are twice asmany blue marbles as green marbles
2.There is a 3/5 probability that the marble chosen will not be blue.
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CEO
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Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
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Re: probability that he will select a red marble greater [#permalink]  27 Jul 2009, 07:11
Expert's post
C

1) it is insufficient. For (4 blue, 2 green, and 1 red) the probability is 1/7 < 1/4 but for (4 blue, 2 green, and 6 red) the probability is 1/2 > 1/4.

2) In other words, the probability of getting blue marble is 1-3/5 = 2/5. The probability of getting red could range from 1/20 (1 red marble from 20 marbles) to 11/20. Insufficient.

1)&2) 2/5 for blue, 1/5 for green and 1-2/5-1/5 = 1/5 for red. Sufficient.

I don't know why we need <=20 marbles here. Maybe I missed something....
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Manager
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Re: probability that he will select a red marble greater [#permalink]  27 Jul 2009, 09:47
1 and 2 are insufficient for the resons cited above.
1+2) Probability of getting a blue marble = 2/5
From 1) There are twice the number of blue marble as green marble,
probabiliyt of getting a green marble = (1/5) - has to be half

Probability of picking a red marble = 1 - (2/5)+ (1/5) = 1- (3/5) = 2/5 which is > 1/4 - no need to solve it but we know that this is sufficient.
Manager
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Re: probability that he will select a red marble greater [#permalink]  27 Jul 2009, 22:56
So the mention of numbers is only to deceive...
Re: probability that he will select a red marble greater   [#permalink] 27 Jul 2009, 22:56
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