This fifth and toughest addition to the successful Times Killer Su Doku series will test your skills to the limit, adding the challenge of arithmetic and taking Su Doku to a new and even deadlier level of difficulty. The 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. With 150 new Tough and Deadly Killer Su Doku puzzles, there is no chance to ease yourself in with easy puzzles. For those who like to live dangerously and push beyond their mental comfort zone, steel yourself for The Times' toughest collection yet.