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Probability - GMATPrep

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by kwah » Thu Feb 23, 2012 3:20 pm
Attached is a probability question from GMATPrep Test.

Answer: C

Can someone please explain the steps to achieve this result?

Thank you,
K
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by pemdas » Thu Feb 23, 2012 4:01 pm
blue balls=12
yellow balls=y
total=12+y
If P(blue ball)<2/5, then P(yellow ball) > 3/5

the least number of yellow balls could be > 3(12+y)/5 (note the sign greater, this will be used later in solution). Let's check each answer choice - apply POE (process of elimination)
a) y=17, 3(12+17)/5 isn't integer
b) y=18, 3(12+18)/5=18 fits but we need the value greater, which could be 19
c) y=19 correct
d) y=20, 3(12+20)/5 isn't integer
e) y=21, 3(12+21)/5 isn't integer

c
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by Anurag@Gurome » Thu Feb 23, 2012 8:13 pm
kwah wrote:Attached is a probability question from GMATPrep Test.

Answer: C

Can someone please explain the steps to achieve this result?

Thank you,
K
Total no. of balls = 12 + y
Probability of drawing a blue ball = 12/(12 + y) < 2/5
60 < 24 + 2y
36 < 2y
y > 18 implies y = 19

The correct answer is C.
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by LalaB » Thu Feb 23, 2012 10:04 pm
y/(12+y)>3/5

5y>36+3y
y>18

==========

12/(12+y)<2/5
Y>18
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