Answer
A useful subset of this set is the set of triples (a, b, c) for which a flies between b and c. For example, (Delta, Detroit, New York) is in this subset, but the triple mentioned earlier is not.
Work Step by Step
This is the set of triples (a, b, c), where a is an airline and b and c are cities. For example, (American, Rochester Hills Michigan, Middle town New Jersey) is an element of this Cartesian product. A useful subset of this set is the set of triples (a, b, c) for which a flies between b and c. For example, (Delta, Detroit, New York) is in this subset, but the triple mentioned earlier is not.
