List out all the factors for each number, then find the largest factor that both numbers have in common.

12 = 1, 2, **3**, 4, 6, 12

15 = 1, **3**, 5, 15.

The greatest common factor in this case is 3.

List out all the factors for each number, then find the largest factor that both numbers have in common.

12 = 1, 2, **3**, 4, 6, 12

15 = 1, **3**, 5, 15.

The greatest common factor in this case is 3.

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