The probability of getting

successes in

trials is

This is the binomial probability distribution. It has a finite number,
, of trials. These repeated trials are independent of each other, so we multiply their probabilities. The outcome of each trial is deemed to be either a success, with probability
, or a failure, with probability
. The probability

of a successful outcome is the same for each trial; and therefore the probability

of a failure is as well. There are

ways of exactly

of the

trials being successes and the probability of

successes is
. Then there are

failures with probability
.
Thus the probability of getting exactly

successes is
.