# Math Help - Figuring out PMF's. HELP!

1. ## 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?

2. $P(X=k)=\frac {{{40} \choose {k}}{{20-k} \choose {10-k}}} {{{60} \choose {10}}}$ where $k=0,1, \cdots,10$

3. Thanks a lot. How do I derive the mean and variance of X for this?

4. Just use the definitions.
Mean: $\sum\limits_{k = 0}^{10} {kP(k)}$.
You will a good calculator or computer algebra system to do that.