Prime Numbers Practice
Practise telling prime numbers from composite ones — a quick yes/no skill that powers factoring and fractions.
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What makes a number prime
A prime number has exactly two factors: 1 and itself. A composite number has more than two. So 7 is prime (only 1×7), but 12 is composite (1, 2, 3, 4, 6, 12).
- 1 is neither prime nor composite — a special case.
- 2 is the only even prime; every other even number is composite.
- To test a number, check whether any number besides 1 and itself divides it — the divisibility rules make this fast.
Worked examples
Tips & common mistakes
You only need to test divisors up to the square root of the number — once you pass it, there’s nothing new to find. The divisibility rules (2, 3, 5…) are your fast first checks. Tap Yes or No.
- Calling 1 prime — it isn’t (a prime needs exactly two different factors).
- Forgetting that 2 is prime — it’s the only even one.
- Assuming odd means prime — 9, 15 and 21 are all odd but composite.
Frequently asked questions
What is a prime number?
A whole number greater than 1 with exactly two factors: 1 and itself. Examples are 2, 3, 5, 7 and 11.
What is a composite number?
A whole number with more than two factors, like 4, 6, 8 and 9. It can be broken into smaller factors.
Is 1 prime?
No — 1 has only one factor (itself), so it is neither prime nor composite.
Is 2 prime?
Yes — 2 is the only even prime number. Every other even number is divisible by 2 and so composite.
How do I check if a number is prime?
Test whether any number other than 1 and itself divides it, using the divisibility rules. You only need to check up to its square root.