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Prep Test 2

by Oxana » Mon Sep 01, 2008 6:31 pm
a contest will consist of n questions, each of which is to be answered either "true" of "false". anyone who answers all n questions correctly will be a winner. What is the least value of n for which the probability is less than 1/1000 that a person who randomly guesses the answer to each question will be a winner?

5
10
50
100
1000

ans is 10
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by nervesofsteel » Mon Sep 01, 2008 11:46 pm
probability of correct answer in each attempt is 1/2

Now for 5 attempts .. prob is 1/(2^5) > 1/1000
For 10 attempts .. prob is 1/(2^10) <1/1000..

Thus B is the amswer... 10 attempts
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by 4meonly » Wed Sep 03, 2008 8:01 am
The probability to answer all questions in gmat correctly by random guess is
1/5^79... :D
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by alexdallas » Sat Jul 04, 2009 9:36 am
any other way to do this one?
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by Ian Stewart » Sat Jul 04, 2009 5:19 pm
4meonly wrote:The probability to answer all questions in gmat correctly by random guess is
1/5^79... :D
Well, if you don't count the AWA, it's actually 1/5^78 (there are 37 Quant questions and 41 Verbal questions). That improves your odds of guessing everything correctly by 400% -- it's not as bad as you thought!
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com
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