Counting Techniques
Summary

the multiplication rule states that the number of possible outcomes is the product of the outcomes of all stages.  Permutations are a special case when each stage has one less outcome than the stage before it.  It has its application in arranging objects where order is important.  Combinations are yet a special case from Permutations when the order is not important.  It has its application in selecting or choosing objects.                                  L. K. Gaafar