What is the greatest common factor? How do you know when you have found the greatest one?

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Factors of 15 are 1, 3, 5, and 15

Factors of 30 are 1, 2, 3, 5, 6, 10, 15 and 30

The factors that are common are 1, 3, 5 and 15

What is the "Greatest Common Factor" ?

It is simply the largest of the common factors.

The largest of the common factors is 15, so the Greatest Common Factor of 15 and 30 is 15

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