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Homework Help: Probability question

  1. Jan 24, 2010 #1
    in a room there are 3 married couples
    somebody guesses which of the men are married to which of the women.
    X=number of correct guesses.

    find E(x), var(x)

    i tried the following

    X=0 (no correct guesses)

    X=1 (one correct guess)

    X=2 (two correct guesses)

    X=1 (three correct guesses)

    BUT the total
    P(X=0) + P(X=1) + P(X=2) + P(X=3) does not come to 1

    where am i going wrong? there are only 3 couples so he could not guess more that 3 correct couples.
    the probability of guessing a correct couple is 1 in 3
  2. jcsd
  3. Jan 24, 2010 #2


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    Science Advisor
    Homework Helper

    You are assuming the probability of a guess being correct is independent of previous guesses. That's not true. For example, there is no chance at all of getting two guesses correct and the third one wrong.
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