Step 1 :
Divide $ 399 $ by $ 58 $ and get the remainder

$$ 399 : 58 = 6 \text{ ( remainder is } \color{blue}{ 51 } \text{ ) } $$

The remainder is positive ($ 51 > 0 $), so we will continue with division.

Step 2 :
Divide $ 58 $ by $ \color{blue}{ 51 } $ and get the remainder

$$ 58 : 51 = 1 \text{ ( remainder is } \color{blue}{ 7 } \text{ ) } $$

The remainder is still positive ($ 7 > 0 $), so we will continue with division.

Step 3 :
Divide $ 51 $ by $ \color{blue}{ 7 } $ and get the remainder

$$ 51 : 7 = 7 \text{ ( remainder is } \color{blue}{ 2 } \text{ ) } $$

The remainder is still positive ($ 2 > 0 $), so we will continue with division.

Step 4 :
Divide $ 7 $ by $ \color{blue}{ 2 } $ and get the remainder

$$ 7 : 2 = 3 \text{ ( remainder is } \color{blue}{ 1 } \text{ ) } $$

The remainder is still positive ($ 1 > 0 $), so we will continue with division.

Step 5 :
Divide $ 2 $ by $ \color{blue}{ 1 } $ and get the remainder

$$ 2 : \color{blue}{ \boxed { 1 }} = 2 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 1 }} $.

This solution can be visualized using a Venn diagram.

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

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