The ones that are relatively prime, or co-prime.
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The factor pairs (not the GCF) of 16 are (16,1)(8,2)(4,4)
The GCF and LCM of 10 and 10 is 10.
16 and 21
By its very definition, a single number cannot have a greatest common factor. The word "common" is there to indicate common to more than one integer. Of the 11 pairs, triplets and quads that can be drawn from the four numbers, 6 have GCF = 3, 4 have GCF = 6, and 1 has GCF = 15.
The GCF is 1.