Gift a puzzle book stocking filler this ChristmasSpecially compiled to provide the most deadly Su Doku challenge, this 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 Doku puzzles 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 100 Extra Deadly Su Dokus. Are you ready for the challenge?Not for the faint-hearted. The puzzles use the same 9x9 grid as a regular Su Doku, but have an extra mathematical element that multiplies the 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!