Prime Numbers Practice
20 questions · 60 seconds · spot the prime among composites
Free · no login · instant feedback on every answer
Train the fastest useful instinct in number work: telling prime from composite at a glance. Each question shows four numbers — usually one prime hiding among three composites, sometimes the reverse — and the clock rewards students who test candidates with divisibility rules instead of staring.
Primes are the atoms of arithmetic: every whole number is built by multiplying primes together, which is why prime recognition sits underneath factoring, simplifying fractions, and later algebra. The skill being drilled here is really systematic checking — is it even, do its digits sum to a multiple of 3, does it end in 5 — applied quickly enough that numbers like 51 and 91 stop passing as prime.
Tips That Make It Stick
- Run the quick checks in order. Even and bigger than 2? Composite. Ends in 5 or 0 (past 5 itself)? Composite. Digits add to a multiple of 3? Composite. These three tests clear most numbers in a second or two.
- Only 7 is left to try. For numbers under 121, any composite that survives the 2, 3, and 5 checks must be divisible by 7 — that catches the great impostors 49, 77, and 91. If 7 fails too, the number is prime.
- Memorize the famous fakes. 51 is 3 × 17, 57 is 3 × 19, 87 is 3 × 29, and 91 is 7 × 13. These four fool more students than any others, and they show up constantly in this drill.
Frequently Asked Questions
What is the difference between a prime and a composite number?
A prime number has exactly two factors, 1 and itself — like 2, 3, 5, 7, and 11. A composite number has more than two factors, like 12 with its six. The number 1 is neither: it has only one factor, so it fails the definition of prime.
Is 51 a prime number?
No — 51 is 3 × 17. It looks prime because it is odd and not on the familiar times tables, but its digits sum to 6, a multiple of 3, which gives it away. Numbers like 51, 57, and 91 are classic composites in disguise.
How do you quickly check if a number is prime?
Test divisibility by the small primes in order: 2 (is it even), 3 (do the digits sum to a multiple of 3), 5 (does it end in 0 or 5), then 7. For any number under 121, passing all four tests means it is prime — you never need to test past the square root.
📝 Matching Printable Worksheets
Prefer paper practice? These free PDF worksheets cover the same skill — each includes an answer key: