What is a Greatest Common Factor?

It is the largest of the common factors of a set of numbers.For example, the factors of 30 are 1,2,3,5,6,10,15 and 30.And the factors of 12 are:1,2,3,4,6 and 12.So the common factors between 12 and 30 are 1,2,3 and 6.Therefore the greatest common factor (GCF) is 6.