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Homework Help: A minor difficulty in PMF

  1. Feb 23, 2008 #1
    I got a minor stuck here... please help me.

    Ok the stat problem is ... a mail order business has 9 phone lines and x is the number of line in use at a specified time. pmf of X is given:

    x 1 2 3 4 5 6 7
    P(x) .1 .15 .2 .25 .2 .06 .04

    Calculate the probability of certain events

    I didn't have any problem with most events but: (e) between two and four lines, inclusive, are not in use: so would it be:
    p(0) + p(5)+ p(6) = .1 +.06 + .04 = .2 ?

    The answer in the back of the book is .65

    What did I do wrong in here?
    Thanks for yoru time and help!
  2. jcsd
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