Which pair of numbers has a greatest common factor of 1?

Any two prime numbers have a greatest common factor of 1.

Examples: The GCF of 5 and 19 is 1. The GCF of 3 and 7 is 1. The GCF of 23 and 29 is 1.

A Prime number and any other number that is not a multiple of it have a greatest common factor of 1.

Examples: The GCF of 2 and 9 is 1. The GCF of 7 and 16 is 1. The GCF of 13 and 60 is 1. Other pairs of numbers without any prime factors in common also have a greatest common factor of 1.

Examples: The GCF of 4 and 9 is 1. The GCF of 21 and 25 is 1. The GCF of 72 and 77 is 1.