Step 1 :
Divide $ 859 $ by $ 78 $ and get the remainder

$$ 859 : 78 = 11 \text{ ( remainder is } \color{blue}{ 1 } \text{ ) } $$

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

Step 2 :
Divide $ 78 $ by $ \color{blue}{ 1 } $ and get the remainder

$$ 78 : \color{blue}{ \boxed { 1 }} = 78 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

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.

GCD Calculator