

The rules of Sudoku are straightforward:
Simply enter the digits 1 to 9 into the blank spaces according to these rules:
 Every row must contain one of each digit 1 to 9  no duplicates allowed.
 Similarly each column must have all the digits 1 to 9, and
 Every 3x3 square must contain the numbers 1 to 9 when the puzzle is completed.
No mathematical ability is required beyond being able to count from 1 to 9.
Each Sudoku has a unique solution that can be reached logically without guessing.
Be warned it is highly addictive once you get the hang of scanning across and down the grid rows and columns!
There are 6.67 billion trillion possible Sudoku squares with 9 x 9 numbers in them  so we are not going to run out of puzzles any time soon.
The minimum number of filled squares required for a unique solution is 17. The more squares that are filled in initially the easier the Sudoku is to solve. However there are 15 to 20 different factors which determine how difficult any particular puzzle is to solve so there can be quite a variation even within the categories 'very easy', 'easy', 'medium' and 'hard'.



