Arranging Balls into Boxes
This Demonstration provides the answers to a variety of questions of the type, "In how many ways can r balls be placed into n boxes?" You can vary the number of balls and boxes. Options include distinguishable or indistinguishable balls, distinguishable or indistinguishable boxes, and whether or not empty boxes are permitted.

Contributed by: Marc Brodie
Download the CDF file to view the simulation using the free Wolfram CDF player.