Hard printable sudoku
When a single cell is not enough, study candidate groups. If exactly two cells in a row allow only {2, 7}, those digits must occupy the two cells, so remove them from the candidates of other cells in that row.
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When a single cell is not enough, study candidate groups. If exactly two cells in a row allow only {2, 7}, those digits must occupy the two cells, so remove them from the candidates of other cells in that row.
What to check
A pair works only if both cells are limited to those two digits. Two cells with additional options do not justify the elimination. The number of given digits alone does not determine the challenge; their relationships matter too.
Write only what you can justify
Compare the row, column and box before entering a digit. If several options remain, note them as small candidates and return later. An entry is confirmed by the board’s rules, not by making a row look neatly ordered.