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Anonymous
21 is not a Prime number because it is 3 x 7
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They are: 5 7 11 13 17 19 23 29 31 and 37
They are: 23 29 31 and 37 which makes four of them
17 19 23 29 31 37.
23, 29, 31, 37
7, 17, and 37 are all prime numbers.