Step 1 :
Divide $ 324 $ by $ 128 $ and get the remainder

$$ 324 : 128 = 2 \text{ ( remainder is } \color{blue}{ 68 } \text{ ) } $$

The remainder is positive ($ 68 > 0 $), so we will continue with division.

Step 2 :
Divide $ 128 $ by $ \color{blue}{ 68 } $ and get the remainder

$$ 128 : 68 = 1 \text{ ( remainder is } \color{blue}{ 60 } \text{ ) } $$

The remainder is still positive ($ 60 > 0 $), so we will continue with division.

Step 3 :
Divide $ 68 $ by $ \color{blue}{ 60 } $ and get the remainder

$$ 68 : 60 = 1 \text{ ( remainder is } \color{blue}{ 8 } \text{ ) } $$

The remainder is still positive ($ 8 > 0 $), so we will continue with division.

Step 4 :
Divide $ 60 $ by $ \color{blue}{ 8 } $ and get the remainder

$$ 60 : 8 = 7 \text{ ( remainder is } \color{blue}{ 4 } \text{ ) } $$

The remainder is still positive ($ 4 > 0 $), so we will continue with division.

Step 5 :
Divide $ 8 $ by $ \color{blue}{ 4 } $ and get the remainder

$$ 8 : \color{blue}{ \boxed { 4 }} = 2 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 4 }} $.

This solution can be visualized using a Venn diagram.

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

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