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marbles

by daretodream » Fri Feb 19, 2010 3:37 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: — Data Sufficiency |
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by thephoenix » Fri Feb 19, 2010 4:32 am
daretodream wrote: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.
imo 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.
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by shashank.ism » Sun Feb 21, 2010 2:26 pm
daretodream wrote: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.
Bag contains red,green and blue marbles.
Total no. of marbles = 20
St.1) B = 2G but we don't know about red...so insuff.
St.2) p(B') = 3/5 --> p(B) = 1-3/5 = 2/5 = 8/20 so no. of blue marbles = 8. we don't know about red and green ... so insuff.
Combined: Blue = 8 so green = 8/2 = 4 so red = 20 -8 - 4 = 8
so probability that he will red marble = 8/20 = 2/5 > 1/4
so suff. Ans C
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by ajith » Sun Feb 21, 2010 7:33 pm
daretodream wrote: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.
1. Is not sufficient since it doesnt give us anything to estitmate the number of red marbles
2. is not sufficient - it tells us at 2/5 of all marbles are blue but will still don't know about red and green

Combining
2/5 is blue
1/5 is green
2/5 is red

Probability that red marble will be chosen is more than 1/4
[spoiler]
Sufficient, C[/spoiler]
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