🔢 MathGrade 4Lesson 6
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Learn Prime and Composite Numbers

Lesson preview: the Mini Lesson and Vocabulary are open to everyone. Guided practice, exercises, the parent checklist and the printable PDF worksheet come with full access (free 7-day trial or membership).

Learning Objectives

  • 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
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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.
  • This rule is agreed by mathematicians worldwide.
Prime vs Composite comparison tablePrime vs CompositePrimeComposite2 factors onlymore than 2 factors2357114 (1,2,4)6 (1,2,3,6)8 (1,2,4,8)9 (1,3,9)10 (1,2,5,10)Remember: 1 is neither prime nor composite.
Factor diagram: 5 is prime, 6 is compositeWhy 5 is Prime, 6 is Composite5 — PrimeFactors: 1 and 51x5= 5Only 1 way to factor.PRIME!6 — CompositeFactors: 1, 2, 3, 61 x 6 = 62 x 3 = 6More than 2 factors.COMPOSITE!
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Tip: To test if a number is prime, try dividing it by 2, 3, 5, 7. If none divide evenly and the number is less than 121, it is prime.

Vocabulary
Prime number
A whole number greater than 1 that has exactly two factors: 1 and itselfe.g. 7 is a prime number.
Composite number
A whole number greater than 1 with more than two factorse.g. 12 is composite because 1, 2, 3, 4, 6, and 12 are all factors.
Factor
A whole number that divides exactly into another numbere.g. 5 is a factor of 25.
Even number
A number divisible by 2e.g. 2 is the only even prime number.
Odd number
A number not divisible by 2e.g. 7 is an odd prime number.
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