Rules
Prime Number Check is a two-player race to spot the prime numbers on a board. Most of the numbers look like they could be prime, and the most valuable one is hidden face down in the middle.
The board
The board is a 5x5 grid of 25 number cubes, all from one range shown to both players, such as 500-570. The centre cube is face down and hides a prime number from that range.
A turn
- You have 30 seconds.
- Name a number on the board that you think is prime.
- If you are right, you score 1 point, the cube is yours, and you can keep naming primes while your time lasts.
- If you are wrong or run out of time, you lose nothing, but the turn passes to your opponent.
Naming the hidden centre prime correctly is worth 3 points.
End of the game
The first player to reach 6 points wins the round. Because a turn only stops at a mistake, scores can go past 6. The match is best of three rounds, each with a new range and a new board. In round 1 the higher-ranked player picks who goes first. After that, the loser of the previous round picks.
How the boards were built
On the show, every number on the board was odd and none ended in 5. The decoys were composites with no small factors, such as 527 (17 × 31), 529 (23 × 23) and 1147 (31 × 37). On the 1100-1180 board, every prime in the range was on view except the hidden one, 1163.
Our version
The board
- Every board is a range that starts on a multiple of 10, such as 510-580. Every prime in the range is on the board except the hidden one in the centre. The other cells are composites from the same range, all odd and none ending in 5.
- The number beside the range is the count of primes in it, the hidden one included. That is how the show’s 9 next to 1100-1180 adds up.
- Numbers are shuffled. Turn on “Sort the board” in Settings to see them in ascending order, which makes the missing prime easier to spot.
- Board size: 2-digit numbers (easy), 3-digit (medium) or 4-digit (hard). On 4-digit boards the decoys have no factor below 7, like the show’s 1147 (31 × 37).
- Tap a number to name it. Type the hidden prime on the keypad. A typed number that is on the board counts as naming that number.
- A number named wrongly is crossed out with its factors, so nobody names it twice.
Solo (Daily and Practice)
- Find every prime on the board and the hidden one. Your score is your time.
- Each wrong number or wrong hidden guess adds 10 seconds.
- The Daily board is 3-digit and the same for everyone each day. Practice boards can be raced online in a room.
Vs AI and online (show rules)
- Each turn lasts 30 seconds in total. A correct prime scores 1 and you keep going. A wrong number, a wrong hidden guess or running out of time ends your turn with no points lost.
- The hidden prime scores 3 and your turn goes on. It does not end the round by itself.
- A round ends the moment someone reaches 6 points. If every prime, the hidden one included, is claimed first, the higher score wins the round.
- Four turns in a row that run out of time also end the round, again to the higher score.
- A tied round that ends early goes to whoever found the hidden prime, or else to the player who did not start.
- Best of three, with a new range and board each round. A coin flip picks who starts round 1; after that the loser of the previous round starts (on the show the loser chose).
- The AI sees the same board you do. Easier levels are slower and more often mistake a composite like 527 (17 × 31) for a prime.
Open questions
The show never explained the number shown beside the range. Counting the primes in range, hidden one included, matches the one example we have, so that is what it means here.
Strategy
Test small primes quickly. Every number is already odd and not a multiple of 5, so start with 3 (add the digits), then 7, 11 and 13. These knock out a lot of decoys.
Know your squares. Decoys like 529 are squares of primes (23 × 23). Learning the squares of primes up to about 40 catches many of the hardest ones.
Go to the square root and stop. To check a number, you only need to try prime factors up to its square root. For numbers below 1,200 that means primes up to 31.
Hunt the hidden prime by elimination. If every prime in the range is on the board except one, the missing one is in the centre. Counting the primes you know against those on view can be worth 3 points.
Bank sure points first. Your turn ends at the first mistake. Name the primes you are certain of before you try the doubtful ones.
Use your opponent’s turn. While they are naming numbers, keep checking the board yourself, so you start your turn with a list ready.