Consider $n$ independent trials of an experiment in which each trial has two possible outcomes, success or failure. The probability of a success on each trial is $p$ and the probability of a failure is $q=1-p .$ In this context, the term $C_{k} p^{k} q^{n-k}$ in the expansion of $(p+q)^{n}$ gives the probability of $k$ successes in the $n$ trials of the experiment.
The probability of a sales representative making a sale with any one customer is $\frac{1}{3}$ . The sales representative makes eight contacts a day. To find the probability of making four sales, evaluate the term
$_{8} C_{4}\left(\frac{1}{3}\right)^{4}\left(\frac{2}{3}\right)^{4}$
in the expansion of $\left(\frac{1}{3}+\frac{2}{3}\right)^{8}$ .