Book excerpt: Just when you thought it was safe to pick up a pencil Killer Su Doku is based on the original Su Doku grid, with the same rules and numbers—1 to 9—but with an added deadly twist. This time there is an element of arithmetic involved and there are few, if any, clues. The aim is to not only complete every row, column, and cube so that it contains the digits 1 to 9, but to also fill in the outlined cubes so they add up to the same number. Hints to solve the puzzle are hidden in the joined squares where only one combination of numbers applies. In the case of joined squares, if the printed number is 3, it should be 1 and 2 that go into the squares. Likewise, in the case of three joined squares, if the printed number is 6, the only combination possible is 1, 2, and 3. To add one final fiendish level, each puzzle also has a time worked out by its Japanese creators so you can try to beat the clock.