Massachusetts Institute of Technology: Srini Devadas and Eric Lehman's "Counting II"

Read from the beginning through Section 1.2; then study the division rule covered in Section 2. Lastly, study Section 3, which discusses counting elements of the union of sets, extending the addition rule: disjoint sets and, then, non-disjoint sets.

The difference rule is as follows: the number of elements in B - A equals (the number of elements in B) minus (the number of elements in B intersect A). In symbols # (B - A) = # (B) - # (B \cap  A).
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