When the smaller one is a factor of the larger one. (That also means the larger
one is a multiple of the smaller one.) The smaller number is the GCF of both.
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When one of the numbers is a factor of the other.
One
Any positive number can be a GCF. But for each separate set of numbers, there can only be one.
When that number is a factor of the greater one.
The GCF of two even numbers is even.