How to play Strike Sum
The rules
The grid is a square of numbers from 1 to 9. Each row has a target at its end, each column a target below it, and the long diagonal, from the top corner where the rows start to the opposite bottom corner, has a target in that bottom corner, marked with an arrow.
Strike out numbers so that, in every row, every column and the diagonal, the numbers left add up to the target.
Every puzzle has exactly one solution, and logic alone is enough to reach it. You never have to guess.
How to play here
- Strike out a number: tap it once.
- Mark a number you keep: tap it again. The ring is only a note for you; a number you have not struck counts as kept.
- Clear a cell: tap it a third time.
- Many cells at once: drag across them. Every cell you pass gets the state the first cell would get from a tap.
- Keyboard: arrow keys to move, Space or Enter to cycle, X or 1 to strike, O or 2 to keep, Delete, Backspace or 0 to clear.
- Undo and redo: Ctrl+Z and Ctrl+Y (on a Mac: ⌘Z and ⌘⇧Z).
- Hint: each press of Hint tells you more: first where to look, then which technique works there, and finally which number to strike or keep. If there is a mistake on the board, the hint shows you where.
Under each target is the current sum of the numbers not struck. When it matches, the target turns green with a check. When a line can no longer reach its target (the numbers you keep add up to more, or you struck too much), the target turns red with a wavy underline.
Difficulty levels
The level is set by the hardest technique the puzzle requires. Larger grids tend to need harder techniques:
| Level | Grid | Techniques required |
|---|---|---|
| Easy | 5×5 | All or nothing, too big |
| Medium | 6×6 | + One way |
| Hard | 7×7 | + Always the same |
| Expert | 8×8 | + Try it |
Techniques
Call the amount a line still has to lose its excess: the sum of the numbers not struck, minus the target.
All or nothing
If the excess is 0, every open number in the line stays. If the excess equals the sum of the open numbers, they all go.
Too big
A number larger than the excess can’t be struck: the line would lose too much. Keep it.
One way
List the choices of open numbers that add up to the excess. If there is only one, strike those numbers and keep the rest.
Always the same
If there are several choices, a number that is in every one of them is struck, and a number that is in none of them stays.
Try it
Suppose a number stays and follow the easier techniques. If some line can then no longer reach its target, the number must be struck. The same works the other way round.
Tips
- Start with lines whose excess is small: only small numbers can go, so the large ones stay.
- Use the diagonal: it crosses every row and every column, so one deduction there often unlocks several lines.
- Mark the numbers you are sure to keep. The rings make the remaining choices easier to see.
FAQ
- How do I play Strike Sum?
- Every row, every column and the long diagonal has a target. Strike out numbers until the numbers left in each of them add up to its target.
- Is there a new puzzle every day?
- Yes. A new Strike Sum is published every day at each of the four levels: easy, medium, hard and expert. Earlier puzzles are in the archive.
- Do I need an account to play?
- No. You play right in your browser, with no sign-up and no cookies. Your progress is saved in your browser.