Some are, some are not.
21 and 28 are composite.
21 is a composite number, as it can be divided by 7 and 3. The other three numbers are prime numbers and do not divide into anything besides themselves.
21
composite numbers between 11 and 21:12 14 15 16 18
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Prime numbers: 11, 13, 17, 19, 23, 29 Composite numbers: 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30
The prime numbers between 10 and 21 are 11, 13, 17, and 19.
2 3 5 7 11 13 17 19
They are all prime numbers except for 15 21 25 and 27 which are composite numbers
11, 13, 15, 17, 19, 21, 23
9, 11, 13, 15, 17, 19, 21
21 and 23