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Homework Help: Train station Probability help

  1. Feb 5, 2009 #1
    1. The problem statement, all variables and given/known data

    Eighty per cent of all scheduled trains through
    Roma Street station arrive on time. If 10 trains
    go through the station every day, find:
    a the probability that at least 8 trains are
    on time
    b the probability that at least 8 trains are
    on time for 9 out of the next 10 days.

    2. Relevant equations

    P(X = x) = nCxpxqn − x



    3. The attempt at a solution

    P(X>=8)

    n=10

    x=8+9+10

    p=0.8

    q=0.2

    P(X=8)+p(X=9)+p(X=10) = 0.6778

    10C80.880.22 +10C90.890.21+10C100.8100.20

    b I guess I would use:

    P(X=8)+p(X=9)+p(X=10) = 0.6778

    But I am not sure how. Please help!
     
  2. jcsd
  3. Feb 5, 2009 #2
    worked it out:

    set:

    n=10

    x=9

    p= 0.6778

    q=0.3222

    And I get: 0.09728
     
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