There cannot be a common factor of just one number. To be common there need to be at least two numbers.
The greatest factor of 21 and 16 is 21, which is a factor of 21 but not of 16.
The greatest common factor of 21 and 77 is 7.
The common factor of 3 and 21 is 3. *
7 is the greatest common factor of 14 and 21.
The common factors of 21 and 42 are 3, 7 and 21. The greatest common factor (GCF) is 21.
The Greatest Common Factor of 21, 63, 84: 21
The greatest common factor of 21 and 75 is 3.
The greatest common factor of 18 and 21 is 3.
The greatest common factor of 21 and 162 is 3.
The greatest common factor of 21 and 322 is 7.
the greatest common factor for 21 and 24 is 3.
The Greatest Common Factor of 21, 9 is 3.