Extreme Killer Sudoku 1, puzzle 91: step-by-step solution
Step through the whole solution, or skip to the hard parts. Blue digits are placed by the walkthrough; red strike-through marks a candidate being ruled out.
Blue digits are placed by the walkthrough. Red strike-through marks a candidate being ruled out. Cells are named by row and column: R3C5 is row 3, column 5. Arrow keys step too.
The key technique: Coloring/Multi-coloring
Following one digit’s strong links, cells get two colours: one colour is true and the other is false. A cell that sees both colours cannot hold the digit.
Other advanced and expert techniques in this solve: Advanced Innies (3+ sectors), Forcing Chains, Multi-Sector Locking.
Every move above the basics is listed on the killer sudoku techniques page, each with a one-sentence explanation and worked examples from the books.
The hard steps, in text (4)
- Step 14 · Multi-Sector Locking advanced
Rules out 1 from R3C7, R9C9; 9 from R4C6, R5C7, R5C9, R8C6. - Step 17 · Advanced Innies (3+ sectors) advanced
Places 3 in R4C4 and rules out 3 from R1C4, R2C4, R3C4, R4C1 and 14 more; 5 from R3C7, R4C6, R8C6, R9C6; 6 from R3C7, R4C6, R5C6, R5C7 and 1 more. - Step 40 · Coloring/Multi-coloring expert
Rules out 4 from R7C8. - Step 72 · Forcing Chains expert
Rules out 3 from R1C2.
The book
No starting digits at all: just cages and their totals. Sudoku rules, plus arithmetic, plus a lot of nerve.
150 puzzles. Expert and beyond. Puzzles by M. J. Bailey. Paperback, 5 × 8 in, 196 pages, £9.99 / $11.99. Every solution in the book carries a QR code to its own walkthrough, so you can check one step without seeing the whole answer.
Try a free sample puzzle (PDF) · prints on A4 or US Letter
More step-by-step solutions: Extreme Sudoku 1 · Extreme Sudoku 2 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.