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 Post subject: Probability...
 Post Posted: Mon Nov 21, 2011 9:05 pm 
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Q: In a certain bag there are 12 identical blue balls, y identical yellow balls and no other ball. One ball will randomly removed. If the probability is less than 2/3 that the removed ball will be blue, then what is the least number of yellow balls that must be in the bag?

a. 17
b. 18
c. 19
d. 20
e. 21


please need help...unable to solve...


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 Post subject: Re: Probability...
 Post Posted: Wed Nov 23, 2011 2:16 pm 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
How far did you get on this question, and what part(s) of the solution didn’t make sense to you? For that matter, where did this problem come from?

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Tim Sanders
Manhattan GMAT Instructor


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 Post subject: Re: Probability...
 Post Posted: Sat Jan 28, 2012 11:56 pm 
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Students


Posts: 4
I think the probability in your question is wrong.
If the probability is less than 2/3 that the removed ball is blue, then the ratio of yellow balls to the total # of balls should be greater than 1/3. From here:
y/(12+y)>1/3
y>6 so y=7 would be the least possible number of yellow balls

so either I have it wrong or your probability is wrong....


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 Post subject: Re: Probability...
 Post Posted: Thu Feb 02, 2012 7:22 pm 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
indeed. the question as originally written is invalid..

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Tim Sanders
Manhattan GMAT Instructor


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