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Math Help - binomial distribution problem

  1. #1
    Junior Member
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    binomial distribution problem

    A student takes a true-false test. There are 12 questions on the test, of which he knows the answer to 4.
    What are the chances of passing this test? (assuming that 50% is a pass).


    How would i set this equation up?
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  2. #2
    Senior Member
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    He needs 6 correct answers to pass and he already has 4. That means he needs 2 more to pass. It's easier to look at how he can fail. That means no correct answers or only 1 correct answer. The probabilities are:

    Probability of no correct answers:
    8 choose 0 (1/2)^8

    Probability of 1 correct answer:
    8 choose 1 (1/2)^8

    Add these 2 up, subtract the result from 1 and you have the probability he has 2 or more correct answers.
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