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

Prime and composite numbers.

• A prime number is a number that has exactly two factors, i.e. $1$ and the number itself.
• A composite number has more than two factors.

Prime numbers less than $20$ are,

$2,3,5,7,11,13,17,19$

Composite numbers less than $20$ are,

$4,6,8,9,10,12,14,15,16,18$

Hence,

Prime numbers less than 20 are: $2,3,5,7,11,13,17,19$.

Composite numbers less than 20 are: $4,6,8,9,10,12,14,15,16,18$.