## Elementary Algebra

$12$
The factors of $36$ are $\{1,2,3,4,6,9,12,18,36\}$ The factors of $60$ are $\{1,2,3,4,5,6,10,12,15,20,30,60\}$ The factors of $84$ are $\{1,2,3,4,6,7,12,14,21,28,42,84\}$ Out of these $3$ sets, the greatest common factor is $12$