To find the Greatest Common Factor between 12 and 30, we have to express each number in factors.
12: 1, 2, 3, 4, 6, 12.
30: 1, 2, 3, 5, 6, 10, 15, 30.
As you can observe, the greatest factor they have in common is 6.
Therefore, the GCF of 12 and 30 is 6.