21, 42, 63, 84, 105, 126, 147, 168, 189, 210.
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The first 12 multiples of 21 are: 21, 42, 63, 84, 105, 126, 147, 168, 189, 210, 231, 252.
Factors of 21 = 1, 3, 7, 21 Multiples of 21: 21, 42, 63, 84, 105, 126, 147 +21 . . .
Multiples of 3 are 3, 6, 9 and so on. Multiples of 7 are 7, 14, 21 and so on. The common multiples of 3 and 7 include 21, 42, 63 and so on.
There are infinitely many multiples of 9 and 21 so it is impossible to list them. One of their multiples is 9*21 = 189.
The common factors are 1 and 7. (7 = 1 x 7 and 21 = 3 x 7) The only common prime factor is 7, which is also the largest common factor.
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