For this case we have that by definition, the Greatest Common Factor or GFC, of two or more integers is the largest integer that divides them without leaving a residue.
So:
We look for the factors of both numbers:
8: 1, 2, 4, 8
40: 1, 2, 4, 5, 8, 10, 20
It is observed that 8 is common.
So, the GFC of 8x and 40 is 8
Answer:
8
