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Fundamental Counting Principle, Factorial

Fundamental Counting Principle

If an event A can occur in m ways, and an event B can occur in n ways, then the two events A and B can occur together in m × n different ways. In other words, if you can choose one element from the set A with m elements and one element from the set B with n elements at a time such that no two pairs are exactly the same, then such a pairing can occur in m × n different ways. This method of matching the elements of two sets or two types of events is called fundamental principle of counting.