Rule of 45
Intermediate Places a digit Used in: Killer Sudoku
Every row, column, and box totals 45. Add up the cages that cover a region and compare with 45: the difference must live in the one cell that spills in (an “innie”) or out (an “outie”) of the region. A powerful way to crack open a fresh Killer grid.
How to spot it
A region whose covering cages overshoot or undershoot the 45-per-unit total by one cell.
What you can conclude
The difference from 45 is the value of that single innie/outie cell.
Worked example
A box’s cages sum to 40, but the box totals 45, so the one unmatched cell is 45 − 40 = 5.