Sudoku X is an interesting change to the traditional Sudoku puzzle. Not only does each number occur exactly once in each row, each column, each region, the main diagonals must also contain each number exactly once as well. This book contains 200 Mixed puzzles, 4 per page. 50 Easy puzzles. 50 Medium puzzles. 50 Hard puzzles. 50 Extreme puzzles. We are sure you will find these puzzles offer an entertaining yet effective way to exercise your brain and keep your mind sharp. All puzzles have been checked to ensure there is only one valid answer. Find more pencil puzzle books on Brainy's website www.BrainysLogicPuzzles.com