21 is a composite number because it has at least one positive factor other than one or itself.
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number 21 is prime
21 is a composite number, not a prime, having prime factors 3x7.
Who says it is? 21 is a composite number!
21 is composite number that is a multiple of 3. 31 is a prime number.
21 is a Composite Number. A prime number is a whole number that only has two factors which are itself and one. A composite number has factors in addition to one and itself. 21 is divisible by 3 and 7.