To determine if 39 is a prime number or a composite number, we simply look at the factors for 39.
If the factors of 39 are only 39 and 1, then we know that 39 is a prime number. Otherwise, 39 is a composite number.
To explain better, if 39 is only divisible by 1 and 39, then it is a prime number, and if 39 is divisible by more than 39 and 1, then it is a composite number.
The factors of 39 are as follows:
1, 3, 13, 39
Therefore, since 39 is divisible by more than one and itself, the answer to the
question "Is 39 prime or composite?" is highlighted in blue below.
composite
Is 40 prime or composite?
OK. Now you know that 39 is a composite number. Use our logic above and see if
you can figure out if the next number on our list is prime or composite before you check your answer.

