The GCD of given numbers is 11.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}66 =& 2\cdot3\cdot11\\[8pt]55 =& 5\cdot11\\[8pt]\end{aligned}$$(view steps on how to factor 66 and 55. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}66 =& 2\cdot3\cdot\color{blue}{\boxed{11}}\\[8pt]55 =& 5\cdot\color{blue}{\boxed{11}}\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 11 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.