Answer :

8. 56 = 7x8. 64 = 8x8
The common factors of 56 is as follows:
1, 2, 4, 7, 8, 14, 28 and 56
The common factors of 64 is as follows:
1, 2, 4, 8, 16, 32, and 64
As you can see, 8 is the greatest common factor