EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 1, puzzle 150: 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 150 of 150 in Extreme Killer Sudoku 1
Difficulty
Expert 2 (expert and beyond), from the final, hardest third of the book
Key technique
Coloring/Multi-coloring
Hard steps
8 of 89 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: 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) (×2), Forcing Chains, Skyscraper, Sue de Coq, XYZ-Wing.

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 8 · Advanced Innies (3+ sectors) advanced
    Rules out 1 from R4C1, R5C2, R5C4; 2 from R4C1, R5C2, R5C4; 4 from R4C1, R5C4.
  2. Step 11 · Forcing Chains expert
    Rules out 7 from R5C4.
  3. Step 14 · Advanced Innies (3+ sectors) advanced
    Places 4 in R5C2 and rules out 4 from R1C2, R2C2, R3C2, R4C3 and 13 more.
  4. Step 61 · Skyscraper advanced
    Rules out 8 from R3C5.
  5. Step 69 · XYZ-Wing advanced
    Rules out 6 from R3C4, R3C5.
  6. Step 70 · Sue de Coq advanced
    Rules out 9 from R5C9; 3 from R8C7.
  7. Step 71 · Coloring/Multi-coloring expert
    Rules out 5 from R2C7.
  8. Step 76 · Coloring/Multi-coloring expert
    Rules out 8 from R2C7, R3C6.

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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More step-by-step solutions: Extreme Sudoku 1 · Extreme Sudoku 2 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.