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rajt
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PostPosted: Sun May 20, 2007 6:43 pm    Post subject: GMAT prep Reply with quote

How do I solve this?

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jayhawk2001
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PostPosted: Sun May 20, 2007 9:07 pm    Post subject: Re: GMAT prep Reply with quote

Probability of answering true = prob of answering false = 1/2

We are asked to find n where

(1/2)^n < 1/1000

We know (1/2)^10 = 1/1024 < 1/1000

Hence n = 10
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Cybermusings
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PostPosted: Tue May 22, 2007 3:32 am    Post subject: Reply with quote

Consider a simple example; Say there are 4 questions; and each question can have 2 possible answers (i.e. either True or False), total possible outcomes would be 2*2*2*2 (or 2^4); If all the questions were to be answered correctly total outcomes would be 1*1*1*1 (First question – Right, Second question – Right, Third Question – Right and 4th Question Right) or 1 outcome; Hence probability would be 1^4 or 1

Now here the total questions are “n”; so total possible outcomes are 2^n; when all questions are answered correctly total possible outcomes would be 1^n or 1; Hence probability = 1^n / 2^n

Keeping the condition in mind 1^n / 2^n < 1/1000
Or 1 / 2^n < 1 / 1000
Multiplying each side by 1000
1000 < 2^n
2^10 = 1024 and 2^9 = 512; so least value of n to satisfy the limitation would be n=10
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tejpreet
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PostPosted: Tue Jun 05, 2007 9:31 pm    Post subject: Reply with quote

Correct Answer is B 10.
And i guess the explanations are good. I agree with them
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