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If a student answers 40 multiple choice questions, each with four possible answers, by guessing randomly, the probability that she answers at least 15 correctly

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  1. prob(right) = 1/4
    prob(wrong) = 3/4

    prob(at least 15 right)
    = prob(15 right) + prob(16 right) + ... + prob(40 right)
    or
    = 1 - (prob(none right) + prob(1 right) + prob(2 right) + ...+prob(14 right)

    I hope you have lots of patience and don't make any errors.

    I will do one of the steps for you, after that you are on your own.

    prob(12 right)
    = C(40,12)(1/4)^12 (3/4)^28
    = .1046489

    I hope you have a good calculator, on mine I have several memory locations so I can store the individual results and maintain reasonable accuracy

    Who would ask such a question?
    A textbook?

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