Step 1 : Find prime factorization of each number.

$$\begin{aligned}144 =& 2\cdot2\cdot2\cdot2\cdot3\cdot3\\[8pt]279 =& 3\cdot3\cdot31\\[8pt]\end{aligned}$$

(view steps on how to factor 144 and 279. )

Step 2 : Put a box around factors that are common for all numbers:

$$\begin{aligned}144 =& 2\cdot2\cdot2\cdot2\cdot\color{blue}{\boxed{3}}\cdot\color{red}{\boxed{3}}\\[8pt]279 =& \color{blue}{\boxed{3}}\cdot\color{red}{\boxed{3}}\cdot31\\[8pt]\end{aligned}$$

Step 3 : Multiply the boxed numbers together:

$$ GCD = 3\cdot3 = 9 $$

This solution can be visualized using a Venn diagram.

The GCD equals the product of the numbers at the intersection.

GCD Calculator