What is a common factor?

A factor is a number that can be multiplied into another number with nothing left over. The factors of 9 are 1, 3, 9.A common factor is a number that is a factor of two different numbers. The factors of 9 are 1,3, 9 and the factors of 12 are 1, 2, 3, 4, 6, 12. The common factors are the numbers that appear in both lists. so here, the common factors are 1 and 3.