The following are chapters out of a book by Devadas and Lehman.  You will see the centrality of counting, the foundation of combinatorics, for discrete structures in that many of the concepts we have covered earlier in this course.

http://www.saylor.org/site/wp-content/uploads/2011/09/CS202-Counting-1.pdf

http://www.saylor.org/site/wp-content/uploads/2011/09/CS202-Counting2-Srini-Devadas.pdf

http://www.saylor.org/site/wp-content/uploads/2011/09/CS202-Counting3-Srini-Devadas.pdf