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### Course: 4th grade > Unit 6

Lesson 2: Prime and composite numbers# Prime and composite numbers review

Review prime and composite numbers, and try some practice problems.

## What are prime numbers?

Prime numbers are numbers with exactly $2$ factors.

A prime number's only factors are ${1}$ and the number ${\text{itself}}$ .

**prime**number. Its only factors are

*Want to learn more about prime numbers? Check out this video.*

## Practice set 1: Identifying prime numbers

*Want to try more problems like this? Check out this exercise.*

## What are composite numbers?

Composite numbers have more than $2$ factors. A composite number has factors in addition to $1$ and the number $\text{itself}$ .

**composite**number. The factors of

*Want to learn more about recognizing prime and composite numbers? Check out this video.*

*Want more ways to think about prime and composite numbers? Check out this article.*

## Practice set 2: Identifying composite numbers

*Want to try more problems like this? Check out this exercise.*

## Want to join the conversation?

- How did scientists discover about prime numbers

Edit: that was a weird question….(76 votes)- I actually have no idea. But I imagine that a mathematician found out that some numbers can only be divided by one and itself. But I am not sure.(54 votes)

- is 2 the only even prime number(33 votes)
- Yes because all other even numbers are divisible by 2(18 votes)

- One question about this: is 1 a prime or a composite number? Neither of those numbers.(20 votes)
- Its a neither because it is cannot be divisible by 2 natural number or it has only on factor(5 votes)

- Is 1 a prime number or a compsite number?(9 votes)
- It is neither prime nor composite because it's only factor is 1.(19 votes)

- Why does math exsits(4 votes)
- Think about all the things that use numbers. All those things use math. You need math to:

1) Keep score in sports

2) Manage your money

3) Design video games, and play video games.

4) All types of engineering use math. Without math, we wouldn't have bridges, buildings, roads, computers, space ships, and many other things that are designed well and built to withstand what ever stresses are placed on them.

5) Advancements in many sciences required math.

Those are just a few examples.(28 votes)

- Why is 2 the lone even prime number?(8 votes)
- A number is prime only if it can be divided by 1 and itself. Because the only factors of 2 are 1 and 2, 2 matches the definition of a prime number and is, therefore, prime.(15 votes)

- whats the point of learning about prime and composites if no one really uses them outside of school?(11 votes)
- In order to learn how to work with fractions, you need to know the difference between prime & composite numbers. Fractions exist in many real life situations outside of school.(9 votes)

- is 349,860,748,398 a composite or prime?(8 votes)
- Composite. It is divisible by other numbers than 2, such as 3.(5 votes)

- How did prime numbers get the name prime numbers?(10 votes)
- prime means something like "only" or "just one" and prime numbers have no other number they are divisible by but themselves and 1. but all whole numbers are divisible by 1.(3 votes)

- Is 31,21,31,41,51,61,71 and 81 prime numbers?(3 votes)
- All are prime except for 21, 51, and 81 (all of which are divisible by 3).(12 votes)