Finding multiples that go into 86 52 18

It's best to use the lowest number since the GCF will max out on the lowest number's highest factor.

18:
2
3
6
9
18

Let's start from the largest factor:

52/18 = 2.888888888_ (not a factor so 18 is eliminated)

52/9 = 5.7777777777_ (not a factor)

52/3 = 17.333333333_ (nope)

52/2 = 26

The only common factor within all of them is 2.