Step 1 :
Divide $ 900 $ by $ 405 $ and get the remainder

$$ 900 : 405 = 2 \text{ ( remainder is } \color{blue}{ 90 } \text{ ) } $$

The remainder is positive ($ 90 > 0 $), so we will continue with division.

Step 2 :
Divide $ 405 $ by $ \color{blue}{ 90 } $ and get the remainder

$$ 405 : 90 = 4 \text{ ( remainder is } \color{blue}{ 45 } \text{ ) } $$

The remainder is still positive ($ 45 > 0 $), so we will continue with division.

Step 3 :
Divide $ 90 $ by $ \color{blue}{ 45 } $ and get the remainder

$$ 90 : \color{blue}{ \boxed { 45 }} = 2 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 45 }} $.

This solution can be visualized using a Venn diagram.

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

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