probability that he will select a red marble

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uptowngirl92: Master | Next Rank: 500 Posts; Posts: 447; Joined: Sun Apr 19, 2009 9:08 pm; Location: Kolkata,India; Thanked: 7 times; GMAT Score:670

by uptowngirl92 » Sat Jul 25, 2009 2:51 am

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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Source: Beat The GMAT — Data Sufficiency | Sat Jul 25, 2009 2:51 am

nplaneta: Junior | Next Rank: 30 Posts; Posts: 20; Joined: Wed Jul 22, 2009 6:29 am

by nplaneta » Sat Jul 25, 2009 9:07 am

1 alone) No - There could be 20 marbles, 6 of which are green, 12 of which are blue and 2 of which are red. P(Red) = 2/20 < 1/4

2 alone) No - P(Blue) = 2/5, so it is possible that P(Green) = 2/5 and P(Red) = 1/5 < 1/4

1 and 2) YES - P(Blue) = 1-(3/5) = 2/5, so from 1. P(Green) = 1/2*(P(Blue)) = 1/5, so P(Red) has to be 2/5 > 1/4

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gmat740: MBA Student; Posts: 1194; Joined: Sat Aug 16, 2008 9:42 pmFollowed by: 17 members; Location: Paris, France; Thanked: 71 times; GMAT Score:710

by gmat740 » Sat Jul 25, 2009 7:51 pm

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?

why are you considering 20 marbles as total?
Please see the BOLD part.

If you consider fewer than 20, then the total, which occurs in the denominator,will decrease and hence the probability will increase. So that makes the answer E.

What is the source of this question. The word "fewer" casts doubt on the feasibility of this question

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DCJ: Senior | Next Rank: 100 Posts; Posts: 68; Joined: Mon May 04, 2009 10:01 pm

by DCJ » Sat Jul 25, 2009 11:55 pm

What is the answer to the question?

The probability of picking a green marble would only be half the probability of picking a blue marble if there were only blue and green marbles in a 1:2 ratio

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