In combinatorics, P(n,k) denotes the number of ways of picking k objects out of n distinct objects.

An example is choosing a class committee consisting of the president, vice-president, treasurer and secretary (4 persons) from a class of 28 students.

Note that order is important, the first one picked will be the president, etc.

The number of ways this can be done is denoted P(28,4), and can be evaluated as:

P(28,4)=28!/(28-4)!=491400.

where P stands for permutation.

Read also the following link for more detailed information:

http://en.wikipedia.org/wiki/Permutation