If an event A can occur in m ways and for each of these m ways, an event B can occur in n ways, then events A and B can occur in m n ways. This counting principle can be generalized to more than two events that happen in succession. So, if for each of the m and n ways A and B can occur respectively, there is also an event C that can occur in s ways, then events A, B and C can occur in m n s ways.
Counting Principle