By Euclid's algorithm, that's the same as the gcf of 28 and 16 - where 16 is calculated as the remainder of 100 divided by 28. (You can continue by doing the division of 28 by 16; the next pair will be 16 and that remainder.)
The greatest common factor of 28 and 100 is 4.
28 = 2 x 2 x 7
100 = 2 x 2 x 5 x 5
The factors they have in common are 2 x 2, so the greatest common factor is 4.
You have :
28 = ( 2 ) ( 2 ) ( 7 )
100 = ( 2 ) ( 2 ) ( 5 ) ( 5 )
The greatest common factor is ( 2 ) ( 2 ) or 4 <--------------
The GCF is 4.
The GCF is 28.
The GCF is 4.
The GCF is 1.
The GCF is 14.
The GCF is 28.
The GCF is 4.
The GCF is 14.
The GCF is 2.
The GCF is 14.
The GCF is 2.
The GCF is 4.