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Quadruple Sudoku

Stage: 3 and 4 Challenge Level: Challenge Level:2 Challenge Level:2
By Henry Kwok

Rules of Quadruple Sudoku

This is a variation of sudoku on a standard 9x9 grid. The object of the puzzle is to fill in the whole 9x9 grid with numbers 1 through 9 (one number per cell) so that each row, each column, and each of the nine 3x3 boxes contains all the nine different numbers 1 through 9.

The clues are in the form of sets of four special clue-numbers. Each set of four numbers can be foundat the intersection of two grid lines and they are the numbers in the four adjacent grid cell.

How the special clue numbers work:
Each set of four cells around four clue numbers overlaps an adjacent set of four cells around a separate set of clue numbers. The shared or overlapping cell must contain a digit that is in both of the small sets.

For example, the two sets ofsmall digits {1678} and {1249} in the top lefthand box of the grid both referencethe cell (2,2). This cell must therefore contain the digit 1 as it is the only digit common to the two sets of special clue numbers.