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Marbles

by 4meonly » Thu Feb 19, 2009 1:56 pm
Oa is D
I got E. Even when stem 1 and 2 are combined we know the probability of either blue (x) or green marble (y) is selected is 12/28.
x + y = 12/28
what is x?
INSUFF


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Last edited by 4meonly on Thu Feb 19, 2009 3:14 pm, edited 2 times in total.
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Source: — Data Sufficiency |
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by mals24 » Thu Feb 19, 2009 2:27 pm
Um you've posted the wrong attachment
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by 4meonly » Thu Feb 19, 2009 3:15 pm
i changed it :)
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by ontopofit » Thu Feb 19, 2009 10:38 pm
stmt 1) (n-2)(n-3)/n(n-1) = 15/28;n=8....suff


2) 2/n(n-1) = 1/28; n = 8...suff

hence D
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by Vemuri » Fri Feb 20, 2009 1:45 am
ontopofit wrote:stmt 1) (n-2)(n-3)/n(n-1) = 15/28;n=8....suff


2) 2/n(n-1) = 1/28; n = 8...suff

hence D
Can you please elaborate? I am not sure I understood how D is the OA.
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by avenus » Fri Feb 20, 2009 8:01 am
ontopofit wrote:stmt 1) (n-2)(n-3)/n(n-1) = 15/28;n=8....suff


2) 2/n(n-1) = 1/28; n = 8...suff

hence D


For the first equation, n=8 and n= 1.62. Since n has to be integer, sufficient.
However, before solving, and just by looking at the equation, you can see that both values will be positive, what could lead you to doubt if this is sufficient. Is there a way to find out quickly, without having to solve (as this would save you precious time, particularly in this one)???
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by ontopofit » Fri Feb 20, 2009 9:42 am
for this type of pbality questions one should know that u dont have to solve this completely...... in DS look for option which gives hint for either both events happening or neither happening coz tat wont take much time to solve,which is 2) in this case......if u get solution u can plug in value for that in the next equation.....which would save much time.....bcoz mostly GMAT presents DS question for which the answer from the both statements will be same.......

hope that helps
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by Vemuri » Sat Feb 21, 2009 12:22 am
Can someone explain how to solve this probability question? I know that in DS question its not required to solve the problem completely, but I want to know how to solve it.

Thanks for your explanation.
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