Answer
$6$
Work Step by Step
When an integer numbers is divided by itd factors it will produce a new integer. An example of this is $20\div 2=10$.
The factors of $12$ are $\{1, 2, 3, 4, 6, 12\}$ and the factors of $18$ are $\{1, 2, 3, 6, 9, 18\}$. The greatest common number in those sets is $6$ as it is the biggest factor that both $12$ and $18$ share.
$1,2,3$ and $6$ are factors that both $ 12$ and $18$ share but $6$ is the biggest number in that set of numbers.
