01 Key Concepts
Prime Number
A whole number greater than 1 with exactly two factors: 1 and itself. Examples: 2, 3, 5, 7, 11, 13.
Composite Number
A whole number greater than 1 with more than two factors, meaning it breaks down into smaller factors.
The Number 1 is Special
1 is neither prime nor composite, since it has only one factor (itself).
The Only Even Prime
2 is the only even prime number — every other even number is divisible by 2, giving it a third factor.
Quick Primality Check
To check if a number is prime, test whether any prime number up to its square root divides it evenly; if none do, it's prime.
02 Solved Examples
Example 1 Is 13 a prime number?
- Square root of 13 is about 3.6, so test primes up to 3: 2 and 3.
- 13 is odd (not divisible by 2). 1+3=4, not divisible by 3.
Answer: Yes, 13 is prime
Example 2 Is 27 prime or composite?
- Check divisibility: digit sum of 27 is 9, which is divisible by 3.
- 27 = 3 x 9, so it has more than two factors.
Answer: 27 is composite
Example 3 List all prime numbers between 1 and 20.
- Check each number from 2 to 20 for exactly two factors.
- Primes found: 2, 3, 5, 7, 11, 13, 17, 19.
Answer: 2, 3, 5, 7, 11, 13, 17, 19
03 Practice Questions
1Is 11 prime?
Yes
2Is 21 prime or composite?
Composite (3x7)
3Is 1 prime, composite, or neither?
Neither
4What is the only even prime number?
2
5Is 29 prime?
Yes
📄 Prime Numbers — Downloadable Worksheet
10 questions with a full answer key. Grab the PDF to print, or try the interactive version in your browser.