Step 12 · Expert
What is Rule of 45 (Several Rows or Columns)? How to Solve Killer Sudoku
Applies the Rule of 45 to two or three neighboring rows or columns (or two boxes) at once — 90 for two lines, 135 for three — subtracting the cages that fit inside.
What it is
Use the Rule of 45 on two or three neighboring rows (or columns), or two neighboring boxes, at once. Two rows add up to 90, three to 135.
When cages stick out of a single row, taking two rows together often lets more cages fit completely, leaving only a few cells to work out.
How to spot it
Look for cages that straddle two rows (or columns). Take both together and subtract the cages that fit inside from 90.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
Each row, column, and box holds 1–9 once, so the numbers in row 7, row 8, and row 9 add up to 135. The 8 cages lying completely inside total 128, so the remaining cells (R7C1 and R7C9) add up to 135 − 128 = 7. R7C1 already holds 6, so R7C9 must be 1. That leaves just one cell, so R7C9 is 1.
→ R7C9 is 1.