21 is composite.
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15 is composite number 17, 19 and 23 are prime numbers.
the composite number is 23
21
18 (All even numbers except 2 are composite.)
15 has more than exactly two factors (1, 3, 5, and 15), therefore it is composite. 17, 19, and 23 each have exactly 2 factors (1 and the number itself), so they are primes.