You need to identify the factors of 20 such that:

`20:{+-1;+-2;+-4;+-5;+-10;+-20}`

You need to identify the factors of 40 such that:

`40:{+-1;+-2;+-4;+-5;+-8;+-10;+-20;+-40}`

You first need to identify the common factors, such that:

`{+-1;+-2;+-4;+-5;+-10;+-20}`

** You need to notice that the greatest common factor is 20, hence, evaluating the greatest common factor...**

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You need to identify the factors of 20 such that:

`20:{+-1;+-2;+-4;+-5;+-10;+-20}`

You need to identify the factors of 40 such that:

`40:{+-1;+-2;+-4;+-5;+-8;+-10;+-20;+-40}`

You first need to identify the common factors, such that:

`{+-1;+-2;+-4;+-5;+-10;+-20}`

**You need to notice that the greatest common factor is 20, hence, evaluating the greatest common factor of 20 and 40 yields `GCF = +-20` .**