Just pick a number, divide and see if you can factor even more.
Since both are even numbers, choose an even number that both divide by.
Like 6
36/6 = 6
90/6 = 15
Can you factor 6 and 15 even more? Yes, by 3
6/3 = 2
15/3 = 5
Can you factor 2 and 5? No, that’s it.
So GCF is 6 * 3 = 18
Lets check:
36/18 = 2
90/18 = 5
Can you factor 2 and 5 even further? No, that’s it
So GCF is indeed 18
