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Probability HW (check my work & help)

  1. Sep 17, 2005 #1
    Scenario: Mr. X is writing letters to five persons A1, A2, A3, A4, A5. After Mr. X has written them he has to leave the room where the letters and envelopes are. Mr X's son, who can't read, decides to help his dad and puts each letter in different envelopes. What is the probability that:

    a) A1 gets his letter
    P = 1/5.

    b) A1 and A2 both gets their letters
    P = (1/5)(1/4) = 1/20.

    c) A1 gets his letter and A2 doesn't get his
    P = (1/5)(3/4) = 3/20.

    d) at least one of A1, A2, A3 gets his/their letter
    Need help with this one.
    P = 1 - (4/5)(3/4)(2/3) = 1 - 2/5 = 3/5 ?

    e) no one gets their letter
    P = 4!/5! = 1/5.
    Last edited: Sep 17, 2005
  2. jcsd
  3. Sep 18, 2005 #2


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    You nailed em. Congrats.

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