Greatest Common Factor (GCF)

The Greatest Common Factor (GCF) of two numbers is the largest factor common to both numbers. For example, if we list out all of the positive factors of $12$ and $18$, we will see that $6$ is the GCF:

$12$: $1$, $2$, $3$, $4$, $6$, $12$

$18$: $1$, $2$, $3$, $6$, $9$, $18$

When the two numbers are not unreasonably large, we can simply list out all of the positive factors of each to identify the GCF. When the numbers are large, however, we have to use a different trick to find the GFC. That'll come later in your studies.