The common factors of 6, 21, and 39 are 1 and 3
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Since 21 is a factor of 63, all of its factors are common.
The only common factor of all three numbers is 3. (The numbers 21 and 63 share all of the factors of 21 : 1, 3, 7, and 21.)
A factor is a divisor - a number that will evenly divide into another number. One way to determine the common factors and greatest common factor is to find all the factors of the numbers and compare them. The factors of 42 are 1, 2, 3, 6, 7, 14, 21, and 42. The factors of 63 are 1, 3, 7, 9, 21, and 63. The common factors are 1, 3, 7, and 21.
The factors of 63 are: 1, 3, 7, 9, 21, 63 The factors of 96 are: 1, 2, 3, 4, 6, 8, 12, 16, 24, 32, 48, 96 The common factors are: 1, 3
The factors of 42 are: 1, 2, 3, 6, 7, 14, 21, 42 The factors of 63 are: 1, 3, 7, 9, 21, 63 The factors of 105 are: 1, 3, 5, 7, 15, 21, 35, 105 The common factors are: 1, 3, 7, 21 The Greatest Common Factor: GCF = 21