Because 7 is a factor of 28 and 63 and 7 is a Prime number.
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7 is a factor of 28 because 4 x 7 equals 28. 7 is a factor of 63 because 9 x 7 = 63. That makes 7 a common factor of 28 and 63. The factors of 28 that are larger than 7 are 14 and 28. The factors of 63 that are larger than 7 are 9, 21 and 63. Since none of those numbers are the same, that makes 7 the greatest common factor of 28 and 63.
Factors of 28: 1, 2, 4, 7, 14, 28Factors of 63: 1, 3, 7, 9, 21, 63Factors of 49: 1, 7, 49GCF (28, 63, 49) = 7
The GCF of 35, 63, and 84 is 7. The prime factorization of 35 is 5*7. The prime factorization of 63 is 3*3*7 The prime factorization of 84 is 2*2*3*7 The GCF of 35, 63, and 84 is thus 7.
2 x 2 x 7 = 28
28 = 7 x 2 x 2