First, prime factor all of the numbers. Then, collect the factors which appear in all three lists:
28 = 2 x 2 x 7
48 = 2 x 2 x 2 x 2 x 3
64 = 2 x 2 x 2 x 2 x 2 x 2
The only factor which appears in all lists is 2. It appears twice in the first number, so the greatest common factor in the group is 2 x 2 = 4
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