Skill: Determine whether a number is composite

A composite number is a whole number greater than $1$ that has more than two positive factors. Another way to recognize one is to write it as a product of two smaller whole numbers.

Example: Is $(18)$ composite?

1. Find factor pairs for $(18)$.

$$1\times18=18$$ $$2\times9=18$$ $$3\times6=18$$

2. List the factors:

$$1,\ 2,\ 3,\ 6,\ 9,\ 18$$

3. Since $(18)$ has more than two factors, it is composite. It is also composite because it can be written as

$$18=2\times9,$$

where both factors are smaller than $(18)$.

A number with exactly two factors, $1$ and itself, is prime. The number $1$ is neither prime nor composite.