What you are actually practicing
A prime number is a whole number greater than 1 whose only divisors are 1 and itself. That definition is simple; applying it under time pressure is not, because it means testing divisibility rather than recognising a shape.
This game shows four candidate numbers and asks which one is prime. The distractors are composite numbers chosen to look plausible — often odd, often ending in digits that give no clue away — so you have to actually test them.
How the game works
Read the four numbers and tap the prime. A countdown runs, and a miss reveals the answer immediately. Your score and best score stay in this browser only.
Easy keeps the candidates small, where the primes are familiar. Medium goes further. Hard uses the full range, where composites become much harder to distinguish at a glance.
Tests that eliminate composites fast
Even numbers above 2 are never prime. This single rule removes a large share of candidates instantly.
Numbers ending in 5 (above 5) are never prime. They are divisible by 5.
Check the digit sum for 3. If the digits add to a multiple of 3, the number is divisible by 3 and therefore composite. 87 gives 8 + 7 = 15, which is divisible by 3, so 87 is composite.
Only test divisors up to the square root. To test 97, you need only try 2, 3, 5, and 7, because 11 squared is already greater than 97. This is the trick that makes large candidates tractable.
Know the small primes by heart. 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31 — recognising these removes the need to test them at all.
Common mistakes, and the fix for each
Mistake: treating 1 as prime. It is not; the definition requires two distinct divisors. The fix is to remember that 1 is the exception everyone forgets once.
Mistake: forgetting 2 is prime. It is the only even prime. The fix is to treat 2 as a special case rather than applying the even rule blindly.
Mistake: assuming odd means prime. 9, 15, 21, 25, and 27 are all odd and all composite. The fix is the divisibility tests, not the last digit.
Why this is worth practising
Prime factorisation underpins fractions, LCM and GCF work, and later algebra. Recognising primes quickly is the difference between a factorisation taking five seconds and taking thirty.
A short, honest FAQ
Do I need to install anything? No. It runs in any modern browser.
Is there always exactly one prime? Yes. Each problem is built with one prime and three composites.
What age is it for? Roughly grades 5 and up.