What is a shortcut for GCF?

Answer 1

Example: 30 and 42
Factor them.
2 x 3 x 5 = 30
2 x 3 x 7 = 42
Select the common factors.
2 x 3 = 6, the GCF

Answer 2

Any time you see two numbers and one is a factor of the other, the smaller number is automatically the GCF of the problem. Any time you see two prime numbers, the GCF is automatically 1.