University of California, Berkeley: Philip Stark's SticiGui: "Chapter 12: Counting"
Read Chapter 13 for an explanation of the most common theories and formulas in probability. There are several exercises and Java applets embedded in the text that are meant to further reinforce your learning. Do not skip these exercises! For instructions on how to navigate these exercises, see "Special Instructions on SticiGui Exercises and Java Applets” in the "Course Information” section above.