With an added arithemetic challenge, the sixth and toughest Times Killer Su Doku collection will test puzzlers' skills to the limit Taking Su Doku to a new and even deadlier level of difficulty, these puzzles use the same 9x9 grid as Su Doku but with an added mathematical challenge. The aim is not only to complete every row, column, and cube so that it contains the digits 1-9, it is also necessary to ensure that the outlined cubes add up to the same number as well. With 150 new Tough and Deadly Killer puzzles, there is no chance to ease in with easy puzzles. For those who like to live dangerously and push beyond their mental comfort zone--here is the toughest collection yet.