University of Hawaii: G.N. Hile's "Set Cardinality"
Read this page, which demonstrates the basic inclusion/exclusion equation outlined in the title of this subunit. The examples on this webpage are especially interesting; pay attention to example 2, which is about playing cards.
Click http://web.archive.org/web/20151023035726/http://www.math.hawaii.edu/~hile/math100/setsd.htm link to open resource.