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5 questions, get 3 right

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by 4GMAT_Mumbai » Sat Sep 11, 2010 9:31 am
Hi,

Denominator: There are 5 ways to answer each of the 5 questions. Hence, the denominator is 5^5.

Numerator:

Number of ways of choosing the 3 questions in which we are going to be right = 5C3 = 10

Number of ways of answering the questions where we are going to be right = 1 *1 * 1

Number of ways of answering the questions where we are going to be wrong = 4C1 * 4C1 = 16.

Is the answer (10 * 16) / (5^5)? Pls clarify.

The assumption is that one HAS TO answer all the questions.

Thanks.
Naveenan Ramachandran
4GMAT, Dadar(W) & Ghatkopar(W), Mumbai
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by anhe123 » Sat Sep 11, 2010 1:05 pm
I think it is the probability of getting P(3/5) + P(4/5) + P(5/5)

which would be 0.2^3 + 0.2^4 + 0.2^5

This is not a GMAT wuestion, but something I though could be a question and was not sure how to answer
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by frank1 » Sat Sep 11, 2010 9:15 pm
i am bit confused...
but would be 5^5 wrong in this case as we may be able to select 2 options in one question....if it 1/5 in each case...
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by 4GMAT_Mumbai » Sat Sep 11, 2010 11:38 pm
andre.heggli wrote:I think it is the probability of getting P(3/5) + P(4/5) + P(5/5)

which would be 0.2^3 + 0.2^4 + 0.2^5
P(3 correct out of 5) = # of ways of choosing 3 questions * Prob of going right in those 3 questions * Prob of going wrong in the other 2 questions

= 5C3 * (0.2 ^ 3) * (0.8^2) = 0.0512

Hope this helps.
Naveenan Ramachandran
4GMAT, Dadar(W) & Ghatkopar(W), Mumbai
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