Sudoku FlowKiller Sudoku
Updated 2026-10-05
The rules
Fill the grid so every row, column and 3×3 box holds 1 to 9 once. The grid is also divided into cages, each marked with the sum of its digits. The digits in a cage must add up to that sum, and no digit may appear twice in the same cage.
Sources: Wikipedia: Killer sudoku
The 45 rule
The digits 1 to 9 add up to 45, so every row, column and box sums to 45. Subtract from 45 the sums of the cages that lie wholly inside a unit, and you get the total of the remaining cells of that unit, often called innies. When the cages that cover a unit also reach outside it, the cells outside are called outies, and their total is the combined cage sum minus 45.
Sources: Wikipedia: Killer sudoku
Where it came from
Killer Sudoku was developed by the Tokyo puzzle setter Tetsuya Nishio from an idea by his student Miyuki Nisawa. Its Japanese name means sum number place. The Times in London began publishing it in 2005.
Sources: Wikipedia: Killer sudoku
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