What is the common factor of 21 and 28 What are the common factors of 30 and 24, 16 and 35, 16 and 4

What is the common factor of 21 and 28 What are the common factors of 30 and 24, 16 and 35, 16 and 4

The so-called common factor is their common divisor
The common factors of 21 and 28 are: 1,7;
The common factors of 30 and 24 are: 1,2,3,6;
The common factors of 16 and 35 are: 1;
The common factors of 16 and 4 are: 1,2,4