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: \(n(∅) = 0\).