Step 1 :
Divide $ 2205 $ by $ 504 $ and get the remainder

$$ 2205 : 504 = 4 \text{ ( remainder is } \color{blue}{ 189 } \text{ ) } $$

The remainder is positive ($ 189 > 0 $), so we will continue with division.

Step 2 :
Divide $ 504 $ by $ \color{blue}{ 189 } $ and get the remainder

$$ 504 : 189 = 2 \text{ ( remainder is } \color{blue}{ 126 } \text{ ) } $$

The remainder is still positive ($ 126 > 0 $), so we will continue with division.

Step 3 :
Divide $ 189 $ by $ \color{blue}{ 126 } $ and get the remainder

$$ 189 : 126 = 1 \text{ ( remainder is } \color{blue}{ 63 } \text{ ) } $$

The remainder is still positive ($ 63 > 0 $), so we will continue with division.

Step 4 :
Divide $ 126 $ by $ \color{blue}{ 63 } $ and get the remainder

$$ 126 : \color{blue}{ \boxed { 63 }} = 2 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 63 }} $.

This solution can be visualized using a Venn diagram.

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

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