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m25 Q27

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m25 Q27 [#permalink] New post 08 May 2011, 05:49
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A bowl contains red and green chips and no chips of any other color. If two chips are drawn from the basket at random without replacement, what is the probability that both chips will be green?

1): 20% of all chips in the basket are green.
2): The ratio of the number of red chips to the number of green chips is 4:1.

Correct answer is E, but I think its D. can someone pls explain? thx
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Re: m25 Q27 [#permalink] New post 08 May 2011, 06:11
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1) Clearly insufficient. The total number of green chips is unknown. Pls dont be fooled by the %age.

2) Clearly insufficient. Just the ratio is not sufficient in this case. We need the exact number of red and green chips

To illustrate this I will take two cases -

If there are 8 red chips and 2 green chips. Then the P(two green) = 2/10 * 1/9 = 2/90 = 1/45

If there are 16 red chips and 4 green chips. Then the P(two green) = 4/16 * 3/15 = 1/20

So you see as the green chips increase the P(two green) is getting bigger. Which makes sense !

Hence the answer is E. To get a single answer to this question - we need the exact number of red and green chips in the bowl

InspiredAnimal wrote:
A bowl contains red and green chips and no chips of any other color. If two chips are drawn from the basket at random without replacement, what is the probability that both chips will be green?

1): 20% of all chips in the basket are green.
2): The ratio of the number of red chips to the number of green chips is 4:1.

Correct answer is E, but I think its D. can someone pls explain? thx
Re: m25 Q27   [#permalink] 08 May 2011, 06:11
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