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'Colour Islands Sudoku 2' printed from http://nrich.maths.org/
By Henry Kwok
Rules of Colour Islands Sudoku
As in a standard Sudoku, the object of the puzzle is to fill in the
whole 9x9 grid with numbers 1 through 9 so that each row, each
column, and each of the nine 3x3 boxes must contain all the nine
different numbers 1 through 9.
There are three coloured "islands" in the 9x9 grid. Within each
'island' there is an extra rule to apply:
- EVERY group of nine cells that form a 3x3 square within each
'island' must contain the numbers 1 through 9.
For example, take the cell r6c8 which contains the number 9 as the
centre of a 3x3 square within the pink 'island'. The eight
surrounding cells (r5c7, r5c8, r5c9, r6c7, r6c9, r7c7, r7c8 and
r7c9) must contain all the remaining eight digits.