PRICES MAY VARY. Group Sum Sudoku This book contains 100 sudoku puzzles with a fun twist. They follow normal sudoku rules but in addition the digits in squares that border a circled number must sum to that total - take a look at the central box illustrated on the front cover to see how it works. This introduces an element of maths to the solving experience, making it akin to killer sudoku. Solutions are included at the back of the book to help out if you get stuck at any point. Happy solving!