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posted by susan

It is a multiple choice exam with 12 problems, each problem has 5 options. You need to answer 10 correctly to pass, what is the probability that by guessing you will answer at least 10 problems correctly?

  1. bobpursley

    at least 10? that means you can get 2 or less wrong.
    Pr(2 wrong)=(4/5)^2*(1/5)^10
    pr(1 wrong)=(4/5)(1/5)^11
    Pr(0 wrong)=(1/5)^12

    so now, Pr( at least 10 right)=1-pr(2wrong)-pr(1 wrong)-pr(zero wrong)

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