We therefore know that Box 3 is missing 3 which can only go in C7 and Box 7 is missing 5 which must go in G3. Lets add up the clues for complete cages in each box. Both boxes contain complete cages apart from one two cell cage. The two red rings surround boxes 3 and 7. This Killer Sudoku is symmetrical - which will often yield a double Innie and Outie. You will see two examples in the first diagram. I've deliberately split this strategy in the solver between the easier-to-spot single cell Innies and Outies and the multiple cell ones - which require more mental calculation and therefore go further down the strategy order. Innies and Outies occur naturally because cages often line up with the Sudoku elements but in the middle and end game you can split the cages that contain solutions and create new useful blocks that were not available at the start. I'm going to refer to Block as any set of adjacent rows, columns and boxes we can use in concert, but also a block could be just one row, column or box. If we can identify a group of cages which almost covers a unit (or more than one unit) but has one cage either sticking out (an Outie) or poking in (an Innie) we can make some very useful deductions. Since the basis of the puzzle is Sudoku, each row, column and box will eventually have solutions which always add up to 45. Fortunately it only requires simple addition and spotting a certain type of pattern. It is extremely versatile and worth looking out for it at every stage. This is a fundamental strategy for Killer Sudoku puzzles of any difficulty.
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