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'Product Sudoku' printed from http://nrich.maths.org/
By Henry Kwok
Rules of Product Sudoku
Like a conventional Sudoku, this Product Sudoku has two basic rules:
1. Each column, row and $3 \times 3$ subgrid must have the numbers 1 to 9.
2. No column, row or subgrid can have two cells with the same number.
The puzzle can be solved with the help of the numbers in the top parts of certain cells. These numbers are the products of the digits in all the cells horizontally and vertically adjacent to the cell.
A Short Demonstration
The cell in the top left corner of this Sudoku contains the number 20. 20 is the product of the digits in the two adjacent cells, which therefore must contain the digits 4 and 5. The 5 cannot go in the cell below the top left hand corner because 5 is not a factor of 96 (the product shown in the third cell down on the left hand side of the puzzle). Therefore 5 must be entered into the cell to
the right of the cell containing 20 and 4 in the cell below.
A printable version of the problem can be found here