Answer :

ProjectTermina
If a number is prime, that means no number will divide into it except itself and one. Let's try dividing 18 by some different numbers and seeing what we come up with.

18 / 7 = 2.57
18 / 4 = 4.5
18 / 17 = 1.06
18 / 2 = 9 ; this means 2 divides into 18.
And there you have it! 18 is not prime.
AL2006
Let's examine it and find out:

The rule is:   If a number has any other factors
besides '1' and itself, then it's composite.

The factors of  18  are:  1,  2,  3,  6,  9, and  18 .

18 appears to be a composite number.