Nobody goes around with lists of greatest common factors in their head.
When we need to know one, we grab the nearest pencil and work it out.
The factors of 40 are 1, 2, 4, 5, 8, 10, 20, and 40 .
The factors of 36 are 1, 2, 3, 4, 6, 9, 12, 18, and 36 .
The common factors of 35 and 40 are 1, 2, and 4 .
The biggest one is 4 .
