Killer Sudoku combines classic Sudoku with simple arithmetic. The usual 1-through-9 rules still apply: every row, column, and 3×3 box holds each digit exactly once.

On top of that, the grid is divided into dotted cages, each labelled with a target sum. The digits inside a cage must add up to that sum — and no digit can repeat inside a single cage.

Start by scanning small cages (2–3 cells) where the possible combinations are limited. A cage sum of 3 across two cells must be 1+2, for example. Cross-reference those candidates against the row, column, and box constraints to lock in certain placements.

Hard puzzles rely on intermediate techniques — locked candidates, naked pairs, X-wings. Expect to pencil-mark systematically rather than keep candidates in your head.

If a puzzle feels stuck, step away for a minute. The eye catches patterns on a second pass that a long stare never surfaces.