## Elementary Algebra

$18$
The factors of $36$ are $\{1,2,3,4,6,9,12,18,36\}$ The factors of $54$ are $\{1,2,3,6,9,18,27,54\}$ Out of these two sets, the $\text{greatest}$ common element is $18$. Therefore, the $\text{GCF}$ of $36$ and $54$ is $18$.