Step 1 :
Divide $ 198 $ by $ 72 $ and get the remainder

$$ 198 : 72 = 2 \text{ ( remainder is } \color{blue}{ 54 } \text{ ) } $$

The remainder is positive ($ 54 > 0 $), so we will continue with division.

Step 2 :
Divide $ 72 $ by $ \color{blue}{ 54 } $ and get the remainder

$$ 72 : 54 = 1 \text{ ( remainder is } \color{blue}{ 18 } \text{ ) } $$

The remainder is still positive ($ 18 > 0 $), so we will continue with division.

Step 3 :
Divide $ 54 $ by $ \color{blue}{ 18 } $ and get the remainder

$$ 54 : \color{blue}{ \boxed { 18 }} = 3 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 18 }} $.

This solution can be visualized using a Venn diagram.

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

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