### Theory:

The cardinality of a set is the number of elements contained in the set. If $$A$$ is a set, the cardinality of a set $$A$$ is written $$n(A)$$.

For example:

State the cardinality of the following sets.

$$A =$$ {$$2, 3, 4, 5, 12$$}

$$B =$$ {$$a, e, i, o, u$$}

set $$A$$ contains $$5$$ elements, hence $$n(A) = 5$$.

In the same manner $$n(B) = 5$$

Important!
The empty set has cardinality zero: $$n(∅) = 0$$.