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'Twin Corresponding Sudoku III' printed from http://nrich.maths.org/
Invented by Henry Kwok
Rules of Twin Corresponding Sudoku
The object of this sudoku variant is to fill in each twin
puzzle with numbers 1 to 9 so that each row, each column, and each
of the nine 3x3 boxes contain all the nine different numbers 1 to
This variant consists of a pair of linked standard sudoku
puzzles each with some starting digits. To get a complete solution
for the twin puzzles, we have to solve each twin puzzle with the
usual sudoku strategy but we can only get the complete solution of
the twin puzzles by substituting the corresponding values of the
digits from one twin sudoku into the other.
So, for example, we find that in r4c9, that is row 4 column 9,
twin A contains the number 7 and twin B contains the number 4. This
means that wherever 7 appears in twin A, it should be substituted
by 4 in the corresponding position in twin B and wherever there is
a 4 in Twin B there is a 7 in twin A. This immediately gives you
some more starting digits in each twin!