This is known as anunordered partition.

If we have distinct items we can group these intogroups ofkeach in ways.N

For example: suppose we have twenty people to divide into four teams of five each. That can be done in .

To see that think of a list of these people.

Make a string "AAAAABBBBBCCCCCDDDDDD".

The number of ways to rearrange that string is .

To remove the identity of the groups we divide by .