Prime Factor Cannon Game

Rocks are falling on the city, and each one carries a number. Your launcher fires prime numbers, and a rock is destroyed only when the primes you load multiply to its number. It is a fast way to practice prime factorization until the factors of common numbers come to mind at a glance.

How to play

1. Tap a rock to target it, or change the target with the arrow keys. The target has a ring round it, and its number is on a highlighted tag.
2. Tap primes on the pad to load them onto the belt. To target 98, load 2, 7 and 7. Every number that is not itself a prime breaks down into primes on the pad, which runs up to 23.
3. Press FIRE. If the primes on the belt multiply to the rock's number, it explodes. A wrong shot jams the launcher for a moment, and each misfire in a row jams it for longer.
4. If the target is a prime, such as 127, press P to fire a prime missile instead. It carries the target's own number, so it only hits a prime. The belt has to be empty.
5. The game ends when the city falls. A wave with no misfires and no buildings lost rebuilds one fallen building.

On a keyboard, 2, 3, 5 and 7 load, T opens a box for typing any other number, Backspace unloads, Enter fires, P fires a prime missile, and Escape or Space pauses. Practice mode shows the product of the belt as you load it, and the city takes no damage, so the game goes on until you end it from the pause screen.

Scoring

Each rock you destroy scores its number times your combo.

• The combo starts at ×1 and goes up by one with every hit, so a run of hits is worth more and more. The COMBO counter shows what your next hit will be multiplied by.
• A misfire resets the combo to ×1. It costs no points, but the launcher jams for a moment. A rock reaching the city resets the combo too.
• A prime missile scores the same way as any other hit.
• The numbers grow every few waves, so most of a high score comes from reaching later waves with a long combo.

For example, destroying 12 at ×1 scores 12, then 45 at ×2 scores 90, and 98 at ×3 scores 294, for a total of 396.

Your five best scores at each speed are saved in this browser. Speed does not change the points, so each speed keeps its own list. Practice games are not saved.

Need a reference? See our list of prime numbers, or try our other free memory games.