Step 1 :
Divide $ 300 $ by $ 155 $ and get the remainder

$$ 300 : 155 = 1 \text{ ( remainder is } \color{blue}{ 145 } \text{ ) } $$

The remainder is positive ($ 145 > 0 $), so we will continue with division.

Step 2 :
Divide $ 155 $ by $ \color{blue}{ 145 } $ and get the remainder

$$ 155 : 145 = 1 \text{ ( remainder is } \color{blue}{ 10 } \text{ ) } $$

The remainder is still positive ($ 10 > 0 $), so we will continue with division.

Step 3 :
Divide $ 145 $ by $ \color{blue}{ 10 } $ and get the remainder

$$ 145 : 10 = 14 \text{ ( remainder is } \color{blue}{ 5 } \text{ ) } $$

The remainder is still positive ($ 5 > 0 $), so we will continue with division.

Step 4 :
Divide $ 10 $ by $ \color{blue}{ 5 } $ and get the remainder

$$ 10 : \color{blue}{ \boxed { 5 }} = 2 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 5 }} $.

This solution can be visualized using a Venn diagram.

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

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