Box Brain Sudoku
Learn › The 45 Rule
Medium · technique 12 of 18
The 45 Rule
Every row, column and box adds up to 45. Also called innies and outies.
A Killer sudoku technique.
How it works
Add the cages that almost exactly cover a house. The gap from 45 is the value of the cell poking in or out.
A worked example
45 rule: the cages touching box 8 sum to 51 but the area sums to 45, so the one cell sticking out, in row 7, column 3, must be 6.
- The cells that make the pattern
- The digit it places
A real position from a puzzle in the app. Small digits are the candidates the step looks at; coloured digits are already solved.
Practise it in Box Brain Sudoku
The app teaches The 45 Rule on this same board, then hands you puzzles that need it. When you are stuck mid-game, a hint first points you to the right area, then names the technique and shows the move.