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.

Medium puzzles introduce situations where straightforward scanning stalls and you need slightly deeper reasoning: hidden singles inside a row or box, pointing pairs, and the occasional two-step deduction.

Pencil marks become useful here. Jot down every candidate for a tricky row, then look for digits that can only go in one cell.