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21 has the factors 21,1,7,3. A Prime number has only 2 factors which are 1 and itself. Composite numbers are everything else except 1 and 0. 1 and 0 are neither prime, nor composite.
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Earn +20 ptsQ: Why is 21 composite and not prime?
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The factors of 21 are 1, 3, 7, and 21 which means that 21 is composite.
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