Click every prime before the clock runs out.
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A grid fills with numbers and you click only the primes. Every correct prime scores; every composite you click costs you. The clock does the rest.
Speed comes from knowing what to rule out rather than from testing each number. Anything even is out except 2. Anything ending in 5 is out except 5. Anything whose digits sum to a multiple of 3 is out except 3. That eliminates most of the grid before you have thought about a single division.
This version: The classic range. Your best score is saved in this browser, so you can come back and try to beat it. Nothing is uploaded and no sign-up is needed.
No. A prime has exactly two distinct factors, and 1 has only one. It is excluded by definition, not by accident — treating 1 as prime would break the unique factorisation theorem.
Only up to the square root. To check whether 91 is prime you test up to 9, and 7 × 13 = 91 turns up quickly. Beyond the square root you would only find factors you have already seen.
They are the multiplicative building blocks of every whole number, and modern encryption rests on the fact that multiplying two large primes is easy while undoing it is not.