EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 2, puzzle 90: 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 90 of 150 in Extreme Killer Sudoku 2
Difficulty
Expert 2 (expert and beyond), from the middle third of the book
Key technique
Coloring/Multi-coloring
Hard steps
9 of 79 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: Empty Rectangle (×3), Advanced Innies (3+ sectors), Forcing Chains, Multi-Sector Locking, Skyscraper.

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 (9)
  1. Step 8 · Multi-Sector Locking advanced
    Rules out 1 from R1C6, R2C6, R6C6, R7C6 and 2 more.
  2. Step 9 · Advanced Innies (3+ sectors) advanced
    Places 3 in R4C5, 9 in R5C5 and rules out 3 from R1C5, R2C5, R3C5, R4C1 and 9 more; 9 from R1C5, R2C5, R3C5, R4C4 and 11 more; 7 from R5C1, R5C2, R5C9.
  3. Step 21 · Empty Rectangle advanced
    Rules out 1 from R1C3.
  4. Step 23 · Skyscraper advanced
    Rules out 1 from R3C1.
  5. Step 24 · Empty Rectangle advanced
    Rules out 2 from R1C3.
  6. Step 26 · Coloring/Multi-coloring expert
    Rules out 5 from R3C5.
  7. Step 27 · Forcing Chains expert
    Rules out 2 from R2C3.
  8. Step 61 · Empty Rectangle advanced
    Rules out 8 from R7C6.
  9. Step 62 · Coloring/Multi-coloring expert
    Rules out 4 from R8C7.

The book

Volume two: 150 new killers, none repeated from book 1. No starting digits, just cages and their totals, ramping from hard to brutal.

150 new 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 Killer Sudoku 1 · Extreme Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.