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30 April, 19:35

According to the record of the registrar's office at a state university, 35% of the students are freshman, 25% are sophomore, 16% are junior and the rest are senior. Among the freshmen, sophomores, juniors and seniors, the portion of students who live in the dormitory are, respectively, 80%, 60%, 30% and 20% If a randomly selected student lives in the dormitory, what is the probability that the student is a freshman (tip: Bayes theorem) ? a. 0.331 b 0.439 c. 0.638 : d. 0.532

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  1. 30 April, 23:30
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    The probability that the student is a freshman is 52.32% If a randomly selected student lives in the dormitory. The right answer is d.

    Explanation:

    According to the data we have the following:

    p (freshman) = 0.35

    p (sophomore) = 0.25

    p (Junior) = 0.16

    p (senior) = 0.24

    Also:

    p (Dormitory/freshman) = 0.8

    p (Dormitory/sophomore) = 0.6

    p (Dormitory/Junior) = 0.3

    p (Dormitory/Senior) = 0.2

    Therefore, to calculate the probability that the student is a freshman If a randomly selected student lives in the dormitory we would habe to use the

    Bayes theorem

    Hence, p (Freshman/Dormitory) = p (Dormitory/freshman) * p (freshman) / {p (Dormitory/freshman) * p (freshman) + p (Dormitory/sophomore) * p (sophomore) + p (Dormitory/Junior) * p (Junior) + p (Dormitory/Senior) * p (senior) }

    = 0.8 * 0.35 / { 0.8*0.35 + 0.6*0.25 + 0.3*0.16 + 0.2 * 0.24}

    = 0.28/0.526

    = 0.532. The probability that the student is a freshman is 52.32%
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