Learn Prime and Composite Numbers
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What you will learn
- 1Define a prime number as a number with exactly two factors: 1 and itself
- 2Define a composite number as a number with more than two factors
- 3Identify prime and composite numbers up to 50
- 4List the prime numbers from 1 to 20
Mini Lesson
Every whole number greater than 1 is either prime or composite. Understanding how numbers divide helps us work with factors, fractions, and algebra.
Prime Numbers
- A prime number has exactly 2 factors: 1 and itself.
- Examples: 2, 3, 5, 7, 11, 13, 17, 19.
- 2 is the only even prime number — every other even number is divisible by 2.
Composite Numbers
- A composite number has more than 2 factors.
- Example: 4 has factors 1, 2, and 4 — three factors, so it is composite.
- Example: 6 has factors 1, 2, 3, and 6 — four factors, so it is composite.
- Example: 9 has factors 1, 3, and 9 — three factors, so it is composite.
The Special Case: 1
- 1 is neither prime nor composite.
- 1 has only one factor — itself. Prime numbers need exactly two.
- Mathematicians around the world agree on this rule.
Words to know
Prime number
A whole number greater than 1 that has exactly two factors: 1 and itself
e.g. 7 is a prime number.
Composite number
A whole number greater than 1 with more than two factors
e.g. 12 is composite because 1, 2, 3, 4, 6, and 12 are all factors.
Factor
A whole number that divides exactly into another number
e.g. 5 is a factor of 25.
Even number
A number divisible by 2
e.g. 2 is the only even prime number.
Odd number
A number not divisible by 2
e.g. 7 is an odd prime number.
Tip: To test if a number greater than 7 is prime, try dividing it by 2, 3, 5, and 7. If none of them divides it evenly and the number is less than 121, it is prime.
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