Sudoku is solved by filling a 9 by 9 grid so every row, column, and 3 by 3 box contains the digits 1 to 9 exactly once. The starting clues restrict which values can go in every empty square.
How to solve a difficult Sudoku
Write candidate digits for difficult squares, then look for singles: squares with one candidate or digits that have only one possible position in a unit. If progress stops, pairs and triples can eliminate candidates from other squares in the same row, column, or box.
Work from confirmed deductions and update candidates consistently rather than guessing. Box-line interactions can help when all possible positions for a digit inside one box lie on the same row or column. If the grid develops a duplicate or a square with no candidate, retrace recent entries: a valid puzzle will not require contradictory placements.
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The numbers solver can help with Countdown arithmetic puzzles.