The GCD of given numbers is 5.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}315 =& 3\cdot3\cdot5\cdot7\\[8pt]55 =& 5\cdot11\\[8pt]\end{aligned}$$(view steps on how to factor 315 and 55. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}315 =& 3\cdot3\cdot\color{blue}{\boxed{5}}\cdot7\\[8pt]55 =& \color{blue}{\boxed{5}}\cdot11\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 5 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.