Killer Sudoku
Classic rules meet arithmetic. Groups of cells — cages — must add up to a target sum, with no repeats inside a cage.
Killer Sudoku combines classic 9×9 Sudoku with arithmetic. As well as filling every row, column, and 3×3 box with 1–9, the digits in each dashed cage must add up to the small number shown, and no digit repeats within a cage. A few givens get you started; the cages do the rest.