Why 45 Matters
The digits 1 through 9 always sum to 45. That means every row, every column, and every 3x3 box in a valid Sudoku must total 45.
Simple Example
If you know a row should add to 45, and the cages inside that row already account for 39, the missing values must total 6.
How to Use It in Killer Sudoku
The 45 rule becomes powerful when cages overlap rows, columns, or boxes. By comparing known cage totals with the total of 45, you can often deduce what is missing from the remaining cells.
Keep It Beginner-Friendly
You do not need heavy arithmetic. Just look for a nearly-complete row, column, or box, add what you know, and compare the remainder against cage targets.