The number of possible combinations of x items in a sample drawn from a batch of size n is:

A combination does not take account of  the order in which the items are drawn from the batch.  Thus if two items are drawn from a batch consists of five items A, B, C, D and E, then AB is the same as BA.

Example

A batch consists of five items A, B, C, D and E. Two items are drawn drawn from this batch, the number of combinations is:

These are:

AB, AC, AD, AE

BC, BD, BE

CD, CE

DE