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Arctic Sudoku
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BasicKiller

Rule of 45 (one cell)

What it is

Every row, column and box sums to 45, so a single cell left outside the cages can be calculated.

How to spot it

Find a box, row or column where the cages lying entirely inside cover every cell but one. Add them up – the difference to 45 is the digit in the last cell.

Example

1231234119135157
  • Basis
  • Removed / placed
The four cages inside box 1 add up to 11 + 9 + 13 + 5 = 38. The box sums to 45, so r3c3 = 45 − 38 = 7.

Used from level: Extra easy (Killer Sudoku)