They are: 12 14 15 16 18 and 20 because they all have more than two factors
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The composite numbers up to 20 are 4,6,8,9,12,14,15,16,18 and 20.
12, 14, 15, 16 and 18.
15 is a composite number - it can be divided by 3 and 5. 11, 13, 17 and 19 are all prime numbers, not composite.
Not all numbers between 20 and 28 are composite numbers. 23 is a prime number.
None of the numbers 7, 11, and 13 are composite numbers; they are all primes.