statistics

a student takes a true false test that has 13 questions and guesses randomly at each answer. Let X be the number of questions answered correctly. Find P(fewer than 4)

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  1. cases:
    one right
    one right
    two right
    three right

    find prob of each case and add them up

    I will do one of them:
    prob(exactly 3 of 13 right)
    = C(13,3) (1/2)^3 (1/2)^10
    = 286(1/2)^13
    = 286/8192
    = 143/4098

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