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Q.

Write down separately the prime and composite numbers less than 20.

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Detailed Solution

Primes < 20 are 2, 3, 5, 7, 11, 13, 17, 19; composites < 20 are 4, 6, 8, 9, 10, 12, 14, 15, 16, 18. The number 1 is neither prime nor composite.

Category	Numbers	Notes
Prime numbers	2, 3, 5, 7, 11, 13, 17, 19	Only 2 factors: 1 and itself; 2 is the only even prime
Composite numbers	4, 6, 8, 9, 10, 12, 14, 15, 16, 18	More than two factors
Neither	1	Special case; not prime nor composite

Check method: Use the sieve of Eratosthenes or test divisibility by small primes (2, 3, 5).

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