What is the biggest number that can go into 40 and 20

So we find what's called the GCF or greatest common factor
firest we find teh factors and group the like ones

40=2 times 2 times 2 times 5
20=2 times 2 times 5
we see a common set of numbers which is in both which is 2 times 2 times 5=20
biggest number that can go into both 40 and 20=20