1 and 2.
2
The first four common multiples of 4 and 7 are 28, 56, 84, and 102
Six
20 has six factors.
Four, four, six, three, six and two factors respectively.
The common factors of six and eight are: 1, 2
Twelve, four and six factors, respectively.
It is six.
4
The greatest common factor is six.
12
Is 12.