The same forced-combination rule on a large cage — where it locks five cells at once.
The forced-combination rule does not stop at pairs. A cage of any size still has a lowest and a highest possible total, and the two sums at each end are still unique — so a big cage sitting on one of them pins every one of its cells in a single stroke. The catch is that the second-from-the-end sums are no longer the obvious set. Five different digits top out at 5+6+7+8+9 = 35; one less than that is NOT '5,6,7,8,9 minus something obvious' — you have to work out which digit gives way. That is what this lesson walks through, and it is where the rule stops being a memory trick and starts being arithmetic.
Killer Sudoku keeps every rule of Classic Sudoku — fill the 9×9 grid so each row, column, and 3×3 box holds the digits 1 to 9 exactly once — and adds cages: dashed groups of cells with a small target sum, where no digit may repeat inside a cage. Puzzles usually start with zero given digits, so every deduction begins from the cage sums. That single extra rule unlocks a whole family of arithmetic techniques.
Practise this in Killer mode on Sudoku Challenge, where the in-game Next Step button walks the exact deduction taught in this lesson.
Explore every Classic and Killer technique in the complete Learning Hub, or read how to play Sudoku to get started.