What Is a Primary Number: Explanation and Examples

How to Tell If a Number Is Primary

A primary number is the same thing as a prime number. Prime numbers are numbers that can only be divided evenly by themselves and one. For example, three is a prime number because it can only be divided evenly by three and one. The number three divided by one equals three (3 ÷ 1 = 3), and three divided by three equals one (3 ÷ 3 = 1). Other examples include 11 and 17.

Any number that is not a prime number is a composite number. The number six is not a prime number because it can be divided by other numbers besides just six and one. For instance, six can also be divided by two and three. Here are some other examples of prime numbers:

• 2
• 5
• 31

Is It Prime?

Now that you know a prime number is a number that can only be divided by itself and one, you can practice with the following examples. If you can think of any other numbers that the number can be divided by - besides itself and one - then that means it is not a prime number.

Example 1: Is 13 a Prime Number?

The number 13 is prime if it can only be divided by 13 and one. Since 13 cannot be divided by any other numbers besides one and 13, this means that 13 is a prime number.

Example 2: Is 15 a Prime Number?

The number 15 can be divided by itself and one, but it can also be divided by three and five. As a result, 15 is not a prime number.

Example 3: Is 19 a Prime Number?

The number 19 can't be divided by any other numbers besides itself and one, so that means that the number 19 is prime.

Example 4: Is 21 a Prime Number?

This number is a prime number because you can divide 21 by one and 21 as well as seven and three.

Example 5: Is 30 a Prime Number?

We know that 30 can be divided by one and 30, but it can also be divided by lots of other numbers like two, three, five, six, ten and 15. As a result, 30 is not a prime number.

Still Having Trouble?

If you're still having trouble figuring out which numbers are prime, it might help to practice your division facts some more. Lots of students have trouble identifying prime numbers because they have difficulty remembering division facts.

It's important to memorize your division facts, but here's a rule that could help you out right away: two is the only even number that is also prime. This means that even numbers are never prime, except for the number two. All even numbers - except for two - can be divided by themselves and one, as well as two and some other number, which means they are not prime. However, because the number two can only be divided by itself and one, it is prime.

Keep in mind that just because even numbers are almost never prime, don't assume that odd numbers are always prime. There are many odd numbers that are not prime too, like 21, 27, 33 and 35.