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tough probability

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by tj123 » Mon Jan 12, 2009 12:47 am
A contest will consist of n questions, each of which can be answer either true or false. anyone who answer all n questions correct is 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?

a) 5
b) 10
c) 50
d) 100
e)1000

b

how to answer this question?
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by adilka » Mon Jan 12, 2009 1:00 am
Q has a 50/50 chance of being right (like a coin toss). so probability of 1 Q being right is 1/2
Probab. of 2 questions being right is 1/2*1/2 or (1/2)^2 = 1/(2^2)
Hence what is the power of 2 that gives you a result >1000

2^2=4
2^4=16
2^8=256
2^9=512
2^10=1024 - Bingo! N=10 is your answer
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by welcome » Mon Jan 12, 2009 5:30 am
1/1000 can be equivalant to 1/2^10.
Shubham.
590 >> 630 >> 640 >> 610 >> 600 >> 640 >> 590 >> 640 >> 590 >> 590
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