Step 1 :
Divide $ 3960 $ by $ 168 $ and get the remainder

$$ 3960 : 168 = 23 \text{ ( remainder is } \color{blue}{ 96 } \text{ ) } $$

The remainder is positive ($ 96 > 0 $), so we will continue with division.

Step 2 :
Divide $ 168 $ by $ \color{blue}{ 96 } $ and get the remainder

$$ 168 : 96 = 1 \text{ ( remainder is } \color{blue}{ 72 } \text{ ) } $$

The remainder is still positive ($ 72 > 0 $), so we will continue with division.

Step 3 :
Divide $ 96 $ by $ \color{blue}{ 72 } $ and get the remainder

$$ 96 : 72 = 1 \text{ ( remainder is } \color{blue}{ 24 } \text{ ) } $$

The remainder is still positive ($ 24 > 0 $), so we will continue with division.

Step 4 :
Divide $ 72 $ by $ \color{blue}{ 24 } $ and get the remainder

$$ 72 : \color{blue}{ \boxed { 24 }} = 3 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 24 }} $.

This solution can be visualized using a Venn diagram.

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

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