Row Reduction puzzles
The Row Reduction method is similar to the checkRows method. To use the Row Reduction method, pick a row, and then pick a number that is missing from that row. Then, test that number in all unfilled cells in that row to see if it is a possibility. If all of the possible cells for the missing number are in the same box, cells in that box that are not in the row cannot contain that particular number, and that candidate can be eliminated as a possibility.
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Using the Row Reduction method, which number goes in cell C4?
Example:
