Box Reduction puzzles
To use the Box Reduction method, begin by choosing a box in the sudoku, and then picking a number that is missing from the box. In that box, if every cell that could contain the missing number is in the same row or column, other cells in that row or column that are in different boxes cannot contain that particular number, because the missing number must be in the box you chose. Therefore, that candidate can be eliminated from other cells in the row or column.
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A
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C
Using the Box Reduction method, which number goes in cell A4?
Example:
