The twelve-cell board in Prime Hunter contains exactly six primes — 2, 5, 11, 17, 23, and 29 — and three wrong picks among the other six cells will end the run permanently. That single sentence carries the entire puzzle, because the game quietly forces you to recognise primes on a fixed grid without a calculator, a cheat sheet, or a countdown timer. You are not searching for unknown primes; you are confirming six known primes and rejecting six specific composites (4, 9, 15, 21, 25, and 33) under a strict three-strike rule. The trick is therefore not mathematical insight but discipline: move with the arrow keys, test with Space or Enter, and avoid the three most common traps — confusing 1 with a prime, trusting the heuristic "odd equals prime", and wasting strikes on the same rejected cell twice. Prime Hunter ships with restart, an exact 880-point clean score, and a boss key, so the learning loop is short and fully observable. This article walks through the board, the definition of a prime, the mistakes that cost strikes, and the score arithmetic so you can finish at 880 on your first attempt.
What the Twelve-Number Board Actually Contains
The board is a three-by-four semantic grid of buttons holding twelve unique integers from 2 through 33. The numbers are not random: the implementation tests them against an exact set, so the winning selection is always the same. Knowing the set removes any doubt about whether 1 might be hiding in a corner — it is not. The cursor always starts on the top-left cell, which shows 2, so your opening move is already decided before the run begins.
| Cell index | Value | Type |
|---|---|---|
| 0 | 2 | Prime |
| 1 | 4 | Composite |
| 2 | 5 | Prime |
| 3 | 9 | Composite |
| 4 | 11 | Prime |
| 5 | 15 | Composite |
| 6 | 17 | Prime |
| 7 | 21 | Composite |
| 8 | 23 | Prime |
| 9 | 25 | Composite |
| 10 | 29 | Prime |
| 11 | 33 | Composite |
The six verified targets live at cell indices 0, 2, 4, 6, 8, and 10, which is why a perfect run is reproducible by clicking or pressing Enter on those exact positions in any order. Every other cell is composite, and three of those composites can each take one strike before the cursor freezes. Because the board is fixed and small, the fastest mental strategy is to memorise the six target indices and walk them in order. The cursor starts on the top-left cell, which is already 2 — your first target — so the opening move of any clean run is simply Space.
Common Mistakes That Burn Your Three Strikes
The strike counter is the heart of the game, and almost every failed run traces back to one of these slips.
Mistake 1 — Treating 1 as prime. The number 1 is not on this board, but the habit leaks in. A prime must be greater than one and have exactly two positive divisors: 1 and itself. Since 1 has only one divisor, it fails both parts of the rule. Wolfram MathWorld's prime number reference repeats the same boundary, and the game enforces it directly: any value below 2 is rejected before the divisor loop runs.
Mistake 2 — Assuming odd numbers are prime. The board contains five odd composites — 9, 15, 21, 25, and 33 — and all five look like prime candidates at a glance. None of them is prime. Each one has a divisor below its square root, so each one fails the same test. The lesson is mechanical: oddness is necessary but not sufficient for primality on this board.
Mistake 3 — Trusting "not divisible by 2, 3, or 5" alone. A passing check against 2, 3, and 5 is enough for most numbers under 30, but it is not the rule the game uses internally. The implementation checks divisors while divisor squared is at or below the candidate, which is sufficient because a composite must have at least one factor at or below its square root. When you eyeball a number, run the same boundary: try 2, then 3, then 5, then 7 only if the candidate is at least 49. On this fixed board, the divisor 7 is never needed, because every composite here is caught by 2, 3, or 5.
Mistake 4 — Pressing Space twice on the same composite. Once a composite cell changes to the rejected state, testing it again is a harmless no-op. The strike counter does not advance a second time and the score is unaffected, so a double-click does not double-punish you. Recognise the rejected colour, move on, and never treat "I might be wrong" as a reason to re-test a cell you have already settled.
Mistake 5 — Forgetting that three distinct composites end the run. Selecting 4, 9, and 15 in any order creates a terminal deadlock. No further arrow or test action can change the result until you press Restart. The deadlock is observable rather than silent, so the failure is honest — but the only escape is a fresh board with the cursor back on 2 and the score reset to 1,000. The same applies to any other three distinct composites on the board.
How to Test a Cell for Primality
This is the routine to run on every cell before you commit. It mirrors what the game's own implementation does and keeps your mental workload small.
- Reject anything below 2. If the highlighted cell shows 0, 1, or a negative number, treat it as composite immediately. On this fixed board you will only see numbers from 2 through 33, so this rule is a quick guard, not a daily check.
- Compute the square root. Take the square root of the candidate. The game stops checking divisors once the divisor squared exceeds the candidate, so anything above the square root is unnecessary to test.
- Test 2 first. If the candidate is even and greater than 2, it is composite. On this board, this single step identifies 4.
- Test 3 next. If the candidate is a multiple of 3, it is composite. On this board, this step identifies 9, 15, 21, and 33 in one sweep.
- Test 5 last. If the candidate ends in 0 or 5 and is greater than 5, it is composite. This identifies 25, which the contract flags explicitly because 5 × 5 = 25.
- Stop at the square root. If no divisor from 2 up to the floor of the candidate's square root divides the number, the cell is prime. On this board, that confirms 2, 5, 11, 17, 23, and 29 without any further work.
- Press Space or Enter only after the test. The game resolves the cell instantly: primes enter the found state, composites enter the rejected state and consume one strike. Already-resolved cells ignore the input.
A clean execution of the prime test on exactly the six target cells produces the 880-point finish. If you would rather watch the board solve itself, open Prime Hunter and replay the 0-2-4-6-8-10 path with arrow keys alone, pressing Space at each target and never on a composite.
Reading the 880-Point Score Equation
The score starts at 1,000 and decreases with every distinct test. The exact formula used by the implementation is:
Score = max(0, 1000 − 20 × attempts − 100 × mistakes)
A clean six-prime run uses exactly six distinct tests and zero mistakes. Substituting the verified numbers directly:
1000 − 20 × 6 − 100 × 0 = 1000 − 120 = 880
That is why 880 is the only "no-mistake" score on this board. Each extra attempt — testing a composite once, then a second composite, then returning to recheck a prime — drops the score by another 20 points. Each composite mistake drops it by another 100. Three mistakes on three distinct composites therefore remove 300 points and finish the run early, well before the formula can fall below zero.
The scoring rewards two habits: think before you press Space, and use arrow keys to skip over obvious composites without testing them. Skipping a cell costs nothing, because skipping does not register as an attempt. The completed best score is stored only in your browser, so a 880-point run is private unless you choose to share it.
Replay Safely With Restart and the Boss Key
Failure is built into the loop, and the controls make recovery quick. Restart restores the cursor to the top-left cell (which always shows 2), clears every found and rejected mark, returns the score to 1,000, and resets the three-strike counter. You can press it after any mistake, after a deadlock, or simply because you want a fresh run.
The boss key is a separate gesture: double-press Escape to swap the screen for a simulated spreadsheet, and double-press Escape again to return to the unchanged board. The board state is preserved across the gesture, so a quick interruption never costs you a strike or a point.
Other controls follow the same accessibility-first design. Arrow keys move among the four columns and three rows. Space and Enter both test the selected cell. Mouse clicks and touch taps do the same on the same numbered buttons. Found, rejected, current, surface, border, focus, and warning states all use the site's CSS variables, so the colour signals are consistent across the site. Every cell announces its value and resolved state to assistive technology, and there is no timer, animation dependency, audio, account, network request, random service, or hardware permission to worry about. The puzzle is fully deterministic: same board, same six targets, same 880 ceiling, every run.
Practise the Same Logic on Sibling Puzzles
If the prime-recognition focus appeals to you, four nearby puzzles use the same compact-keyboard style without overlapping rules. Number Sequence Quiz asks you to spot five arithmetic patterns before two mistakes end the run. Magic Square arranges 1 through 9 so every row, column, and diagonal sums to the same constant. Math Maze applies the four basic operations along a small grid to land exactly on 10. Number Match 10 clears a six-number board by choosing adjacent pairs that sum to 10.
For a broader take on avoiding mistakes in compact number games, the walkthrough on avoiding mistakes in Number Balance covers a similar "tiny board, exact rule, no timer" structure. The pattern generalises well: know the rule, know the losing condition, memorise the winning positions, and let the deterministic scoring confirm every correct choice. With those habits in place, the 880-point finish on Prime Hunter becomes a routine outcome rather than a lucky one.