The GCF can be obtained by taking the prime factors of two numbers then multiplying the common prime factors. IF there are none, then the GCF is 1 and the numbers are called relatively prime.
There are a lot of relatively prime numbers. To name three sets:
3 and 19
7 and 20
15 and 28
For these sets, we see that there are no other common factors except 1. To illustrate the factors:
3 = 1, 3
19 = 1, 19
7 = 1, 7
20 = 1, 2, 2, 5
15 = 1, 3, 5
28 = 1, 2, 2, 7
So we see that only the number 1 is the factor in each set.