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Posted by on Thursday, April 9, 2009 at 11:41pm.

Involving a true-false test. Assume that 8 questions are answered by guessing.

What is the probability of at least 6 correct answers?

I know this a Burnoulli Trial problem and that N=8 and p=.5 and q=.5
I just don't know where to go from there.

  • College Finite Math - , Friday, April 10, 2009 at 12:19am

    Add the probabilities of 6, 7 and 8 correct answers. The probability of getting all 8 right is (1/2)^8 = 1/256
    The probability of getting 6 right is (1/2)^8*8!/[6!*2!] = 56/[256*2] = 28/256
    The probability of getting 7 right is
    (1/2)^8*8!/[7!*1!]= 8/256

    The total probability is 37/256

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