Step 1 :
Divide $ 17990 $ by $ 12990 $ and get the remainder

$$ 17990 : 12990 = 1 \text{ ( remainder is } \color{blue}{ 5000 } \text{ ) } $$

The remainder is positive ($ 5000 > 0 $), so we will continue with division.

Step 2 :
Divide $ 12990 $ by $ \color{blue}{ 5000 } $ and get the remainder

$$ 12990 : 5000 = 2 \text{ ( remainder is } \color{blue}{ 2990 } \text{ ) } $$

The remainder is still positive ($ 2990 > 0 $), so we will continue with division.

Step 3 :
Divide $ 5000 $ by $ \color{blue}{ 2990 } $ and get the remainder

$$ 5000 : 2990 = 1 \text{ ( remainder is } \color{blue}{ 2010 } \text{ ) } $$

The remainder is still positive ($ 2010 > 0 $), so we will continue with division.

Step 4 :
Divide $ 2990 $ by $ \color{blue}{ 2010 } $ and get the remainder

$$ 2990 : 2010 = 1 \text{ ( remainder is } \color{blue}{ 980 } \text{ ) } $$

The remainder is still positive ($ 980 > 0 $), so we will continue with division.

Step 5 :
Divide $ 2010 $ by $ \color{blue}{ 980 } $ and get the remainder

$$ 2010 : 980 = 2 \text{ ( remainder is } \color{blue}{ 50 } \text{ ) } $$

The remainder is still positive ($ 50 > 0 $), so we will continue with division.

Step 6 :
Divide $ 980 $ by $ \color{blue}{ 50 } $ and get the remainder

$$ 980 : 50 = 19 \text{ ( remainder is } \color{blue}{ 30 } \text{ ) } $$

The remainder is still positive ($ 30 > 0 $), so we will continue with division.

Step 7 :
Divide $ 50 $ by $ \color{blue}{ 30 } $ and get the remainder

$$ 50 : 30 = 1 \text{ ( remainder is } \color{blue}{ 20 } \text{ ) } $$

The remainder is still positive ($ 20 > 0 $), so we will continue with division.

Step 8 :
Divide $ 30 $ by $ \color{blue}{ 20 } $ and get the remainder

$$ 30 : 20 = 1 \text{ ( remainder is } \color{blue}{ 10 } \text{ ) } $$

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

Step 9 :
Divide $ 20 $ by $ \color{blue}{ 10 } $ and get the remainder

$$ 20 : \color{blue}{ \boxed { 10 }} = 2 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 10 }} $.

This solution can be visualized using a Venn diagram.

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

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