The GCD of given numbers is 25.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}25 =& 5\cdot5\\[8pt]125 =& 5\cdot5\cdot5\\[8pt]\end{aligned}$$(view steps on how to factor 25 and 125. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}25 =& \color{blue}{\boxed{5}}\cdot\color{red}{\boxed{5}}\\[8pt]125 =& \color{blue}{\boxed{5}}\cdot\color{red}{\boxed{5}}\cdot5\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 5\cdot5 = 25 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.