Answer:
7
Step-by-step explanation:
To find the GCF, or greatest common factor, of a set of numbers, we should list out each of their factors.
21: 1, 3, 7, 21
28: 1, 2, 4, 7, 12, 28
After listing them out, we can tell that 7 is the greatest number that are in both of these lists.
21: 1, 3, 7, 21
28: 1, 2, 4, 7, 12, 28
So, 7 is the GCF of 21 and 28.
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