First we have to identify common factors among these two terms.
40 16 l 2 We can divide both by 2
20 8 I 2 We can divide both by 2
10 4 I 2 We can divide both by 2
5 2 I x There is no number different than one to divide them at the same time
Then , we multiply our factors , doing so, we get 2*2*2=8
X is also a common factor, so the GFC must be 8x.