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The greatest common factor of 7 and 28 is 7.
The greatest common factor of 21 and 77 is 7.
The greatest common factor of 18 , 7 = 1
The GCF of 7, 21, and 42 is 7, because 7 is a factor of all three.
1 is the greatest common factor of 7 and 8. There are no others.