Back to the arcade
30-second rounds · 3 min read
Play Prime TimeWhat is Prime Time?
Classify each integer as prime or composite. A prime has exactly two positive divisors: one and itself. A composite has an additional factor. The game uses integers from two upward, avoiding the special case of one. Quick recognition helps, but a short divisibility check is often safer than relying on whether a number looks familiar.
Rules and controls
- Press Prime or Composite, or use the displayed numbered keyboard shortcut when the stage has focus. A correct classification earns one objective. A wrong answer records a mistake. Both outcomes move to a fresh integer after a brief transition, so repeatedly pressing the same button without reading the next number is unreliable. Waiting on an integer creates no achievements.
- Controls respond only during active play. The countdown and the brief change between boards are pauses, not opportunities to submit the next answer early. Touch controls and native Tab/Enter button activation follow the same replay rules.
Use the controls displayed above the playfield. Touch buttons and keyboard shortcuts are shown before the round begins.
Scoring and rewards
The game scoreboard gives 100 for each correct prime classifications objective and subtracts 25 for a recorded mistake, with a minimum displayed score of zero. Only new, accepted objectives count; waiting or repeating completed work does not create achievements.
Reward points use verified performance against this game’s reference pace rather than its raw scoreboard. The reference is: A considered primality decision every 1.4–2 seconds, plus transition. Full reference performance corresponds to 40 base reward points and 60 XP per minute; the account’s level bonus is applied separately.
Points and XP scale with objective performance, precision, and round length. At full reward performance, a 30-second round earns up to 20 base points and 30 XP. Each level gained adds a 0.1% point bonus. Guest practice has no reward balance.
For eligible rewarded rounds, points remain pending until the sponsored claim’s 12 seconds of focused time and fresh hCaptcha are complete. No ad click or purchase is required.
How difficulty changes
The upper end of the number range grows with daily difficulty. Larger candidates may require testing an additional small divisor, but the definition of a prime never changes. The round still lasts thirty seconds, encouraging a balance between quick recognition and deliberate checking rather than long written calculations.
Difficulty increases after every three starts of this game during a UTC day, up to level 10, and resets at 00:00 UTC. Your player level and its point bonus are separate from the game’s daily difficulty.
Strategy
Plan the next move
Eliminate simple composites first. Every even number greater than two is composite; numbers ending in five or zero are composite unless the number is five itself. A digit sum divisible by three also reveals divisibility by three.
Check before committing
For a candidate that survives those checks, test divisors only up to its square root. If no integer divisor exists by then, a larger complementary divisor cannot appear on its own.
A worked example
- Consider 29. It is odd, does not end in five, and its digit sum is eleven rather than a multiple of three.
- Its square root lies between five and six. Check five as well; 29 is not divisible by five.
- There is no relevant small divisor, so choose Prime.
Common mistakes
- Two is prime, despite being even.
- Nine, twenty-one, and forty-nine are composite even though their final digits are common among primes.
Questions
Will I be asked about one?
No. Candidates start at two, so every displayed number belongs to exactly one of the offered categories.
Can a prime have repeated digits?
Yes. The appearance of its decimal digits does not determine primality; factors do.
