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Math Help - Figuring out PMF's. HELP!

  1. #1
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    Figuring out PMF's. HELP!

    There are 40 State employees
    There are 20 Federal Employees
    A random sample of 10 is to be asked to fill out a benefits questionnaire.

    a) Let X be the number of State workers. Write the pmf of X.

    HOW DO YOU WRITE A PMF/PDF. My teacher is a moron and doesn't teach. Can someone teach me how?
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  2. #2
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    P(X=k)=\frac {{{40} \choose {k}}{{20-k} \choose {10-k}}} {{{60} \choose {10}}} where k=0,1, \cdots,10
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    Thanks a lot. How do I derive the mean and variance of X for this?
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  4. #4
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    Just use the definitions.
    Mean: \sum\limits_{k = 0}^{10} {kP(k)} .
    You will a good calculator or computer algebra system to do that.
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