List the digit sets that can fill a cage, then drop the digits no combination uses.
A cage tells you its total and how many cells it spans — and that already rules out digits. List every way to make the sum with distinct digits; any digit that appears in NO surviving combination cannot go anywhere in the cage.
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.