The GCD of given numbers is 13.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}13 =& 13\\[8pt]39 =& 3\cdot13\\[8pt]\end{aligned}$$(view steps on how to factor 13 and 39. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}13 =& \color{blue}{\boxed{13}}\\[8pt]39 =& 3\cdot\color{blue}{\boxed{13}}\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 13 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.