A number is prime if it has only 1 and itself as factors. For example, 7 is a Prime number because it has only 1 and 7 as factors. 15 isn't a prime number because it has 1, 3, 5, and 15 as factors.
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Composite
15 is a factor of 45, but it is not a prime number.
a prime number is a number that can only be divided by 1and its self... 1 isnt a prime number but 2, 3, 5, 7, 9, 11 etc... are prime numbers.
2 is a prime number and 15 is a composite number.
23, 37, 79 and 51 (assuming 1851 is a typo, 18 isnt a prime) if not a typo 1851 isnt a prime number anyway, it divides by 3