The greatest common factor between any list of numbers is the biggest multiple that they all share.
For example, 5, 35 and 50 all share 5 as the biggest common factor, making 5 the greatest common factor.
Now that we have our example and definition out of the way, let's make out the factors.
12:
1 x 12
2 x 6
3 x 4
40:
1 x 40
2 x 20
4 x 10
5 x 8
60:
1 x 60
2 x 30
3 x 20
4 x 15
5 x 12
6 x 10
Now that we have our factors, let's look for the biggest number that they have in common.
They all share 4 as the biggest number.
Your GCF (Greatest Common Factor) is 4.
I hope this helps!