This tells you that the GCF of 7 and 28 has to be either 1 or 7. The number 28 is a composite number; its factors are: 1, 2, 4, 7, 14, and 28. The only common factors between 7 and 28 are 1 and 7. Since 7 is greater than 1, it's the greatest common factor of 7 and 28. The answer is 7.
