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The odd prime numbers less than 20 are 3, 5, 7, 11, 13, 17, and 19.
The composite numbers up to 20 are 4,6,8,9,12,14,15,16,18 and 20.
15 is a composite number - it can be divided by 3 and 5. 11, 13, 17 and 19 are all prime numbers, not composite.
The first 20 odd numbers are: 13579111315171921232527293133353739.
Not all numbers between 20 and 28 are composite numbers. 23 is a prime number.