Classic 9x9 sudoku at hard difficulty — 21 clues given, 60 cells to fill in. The fewest clues of the three, requiring more advanced deduction.
This preset locks the difficulty to hard, which starts each puzzle with 21 clues (the generator removes 60 of the 81 cells from a completed, valid 9x9 grid). The fewest clues of the three, requiring more advanced deduction. Every puzzle has exactly one solution. Solve it on screen or download a print-ready PDF with an optional answer key.