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Specially compiled to provide the most devilishly difficult challenge, Ultimate killer Su Doku 4 is the only volume for Su Doku enthusiasts who need a puzzle that really tests their mettle.Prepare yourself for the toughest Su Doku challenge there is. These diabolically difficult Ultimate Killer Su Dokus will really put your brainpower to the test as you warm up with the 100 Deadly Killer puzzles before steeling yourself to take on the 20 Extra Deadly Su Dokus. Are you ready for the challenge?The Times Ultimate Killer Su Doku 4 is not for the faint-hearted. The puzzles use the same 9x9 grid as a regular Su Doku, but have an extra mathematical element that makes them a real challenge. The aim is not only to complete every row, column and cube so that it contains the digits 1 to 9, but also to make sure that the outlined sections, called cages, add up to the number given in each cage.Warning: Not suitable for amateur puzzlers.If you survived the first three collections of Ultimate Killer Su Doku, then you might be ready to take on the fourth.