Step 2 · Beginner
What is Naked Single (only one number fits)? How to Solve Killer Sudoku
When a cell has only one candidate left after considering its row, column, box, and cage, that candidate is its answer.
What it is
Pick a cell and cross out every digit already in its row, column, and box. If only one digit is left, that's the answer.
Example: the row has 1–5, the column has 6 and 7, and the box has 8. Only 9 can go there.
How to spot it
Look where busy rows, columns, and boxes cross. Those cells usually have the fewest options.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
R7C6 can only be 7. Already used: 2, 3, 4, 6, 8, and 9 in its row; 3, 6, and 8 in its column; 1, 3, 5, 6, 8, and 9 in its box. Of 1–9, only 7 is left.
→ R7C6 is 7.