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# Product Sudoku

Alison and Charlie kept a record of the order in which they filled the Sudoku.

They filled the cell marked 1 first, then the cell marked 2, then the cell marked 3...

You might like to retrace their route to fill in the cells in the same order, though this is just one possible route through the problem.

*You can print their journey here. *

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Age 11 to 16

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Alison and Charlie kept a record of the order in which they filled the Sudoku.

They filled the cell marked 1 first, then the cell marked 2, then the cell marked 3...

You might like to retrace their route to fill in the cells in the same order, though this is just one possible route through the problem.

Make a set of numbers that use all the digits from 1 to 9, once and once only. Add them up. The result is divisible by 9. Add each of the digits in the new number. What is their sum? Now try some other possibilities for yourself!

Take any pair of two digit numbers x=ab and y=cd where, without loss of generality, ab > cd . Form two 4 digit numbers r=abcd and s=cdab and calculate: {r^2 - s^2} /{x^2 - y^2}.