Common factor of some numbers - a number, which is a factor of each of them. For example, numbers 36, 60, 42 have common factors 2 and 3 . Among all common factors there is always the greatest one, in our case this is 6. This number is called a greatest common factor (GCF).

To find a greatest common factor (GCF) of some numbers it is necessary:

1. to express each of the numbers as a product of its prime factors, for example:
   360 = 2 · 2 · 2 · 3 · 3 · 5 ,
2. to write powers of all prime factors in the factorization as:
   360 = 2 · 2 · 2 · 3 · 3 · 5 = 2³ · 3² · 5¹ ,
3. to write out all common factors in these factorizations;
4. to take the least power of each of them, meeting in the all factorizations;
5. to multiply these powers.