4. Product rule

8. Generalized product rule:
If we have a procedure consisting of sequential tasks T1, T2, …, Tm that can be done in n1,
n2, …, nm ways, respectively, then there are n1 n2 … nm ways to carry out the procedure.

17. K+1
k

19. Example : In our 60-student class, at least 12 students will get
the same letter grade (A, B, C, D, or F).