Step 1 :
Divide $ 1105 $ by $ 780 $ and get the remainder

$$ 1105 : 780 = 1 \text{ ( remainder is } \color{blue}{ 325 } \text{ ) } $$

The remainder is positive ($ 325 > 0 $), so we will continue with division.

Step 2 :
Divide $ 780 $ by $ \color{blue}{ 325 } $ and get the remainder

$$ 780 : 325 = 2 \text{ ( remainder is } \color{blue}{ 130 } \text{ ) } $$

The remainder is still positive ($ 130 > 0 $), so we will continue with division.

Step 3 :
Divide $ 325 $ by $ \color{blue}{ 130 } $ and get the remainder

$$ 325 : 130 = 2 \text{ ( remainder is } \color{blue}{ 65 } \text{ ) } $$

The remainder is still positive ($ 65 > 0 $), so we will continue with division.

Step 4 :
Divide $ 130 $ by $ \color{blue}{ 65 } $ and get the remainder

$$ 130 : \color{blue}{ \boxed { 65 }} = 2 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 65 }} $.

This solution can be visualized using a Venn diagram.

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

GCD Calculator