EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 1, puzzle 124: 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.

Puzzle
Killer sudoku, number 124 of 150 in Extreme Killer Sudoku 1
Difficulty
Expert 3 (expert and beyond), from the final, hardest third of the book
Key technique
Forcing Chains
Hard steps
8 of 67 steps need an advanced or expert technique

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: Forcing Chains

Try each candidate of one cell in turn. Every choice leads to the same result, so that result must be true.

Other advanced and expert techniques in this solve: Multi-Sector Locking (×3), Advanced Innies (3+ sectors) (×2), Finned Fish, Remote Pairs.

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 (8)
  1. Step 7 · Multi-Sector Locking advanced
    Rules out 8 from R3C7; 9 from R3C7.
  2. Step 8 · Advanced Innies (3+ sectors) advanced
    Places 3 in R4C5 and rules out 3 from R2C5, R3C5, R3C7, R4C1 and 11 more.
  3. Step 13 · Multi-Sector Locking advanced
    Rules out 8 from R6C7, R6C8, R6C9; 9 from R6C7, R6C8, R6C9.
  4. Step 14 · Forcing Chains expert
    Rules out 5 from R2C6.
  5. Step 23 · Advanced Innies (3+ sectors) advanced
    Rules out 6 from R3C1; 8 from R3C1.
  6. Step 51 · Multi-Sector Locking advanced
    Rules out 4 from R8C8.
  7. Step 52 · Remote Pairs expert
    Rules out 2 from R6C9, R7C7, R9C9; 6 from R6C9, R7C7, R9C9.
  8. Step 55 · Finned Fish expert
    Rules out 6 from R8C8.

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.

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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.