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: \(n(∅) = 0\).