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Arctic Sudoku
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HardKiller

Innies and outies

What it is

When cages cover a box (or rows or columns) plus a few cells outside, the difference from 45 gives those cells.

How to spot it

Add up the cages that cover a box, row or column. If they also reach a few cells outside (outies) or leave a few inside uncovered (innies), the difference to 45 – or 90 or 135 for two or three units – is their sum.

Example

123123411109205
  • Basis
  • Removed / placed
The four cages cover box 1 plus r3c4. Their total is 11 + 10 + 9 + 20 = 50. The box is 45, so the outie r3c4 = 50 − 45 = 5.

Used from level: Expert (Killer Sudoku)