Sudoku Solving Techniques— Concepts, Formulas & Shortcuts
- A standard Sudoku is a 9 × 9 grid divided into nine 3 × 3 boxes.
- Every row, every column and every 3 × 3 box must contain the digits 1–9 exactly once.
- Naked single: a cell with only one remaining candidate — fill it immediately.
- Hidden single: a digit that can legally go in only one cell within a row, column or box.
- Pointing pair: if a digit in a box is confined to one row or column, eliminate it from the rest of that line.
- A valid Sudoku has exactly one solution; the proven minimum number of clues for uniqueness is 17.
Sudoku Solving Techniques Practice Questions with Answers
Attempt each question first, then open the explanation. All 6 questions below are free to read and require no signup.
Q1.A standard Sudoku grid has dimensions:
Easy- A6 × 6
- B8 × 8
- C9 × 9
- D12 × 12
Sudoku Solving Techniques question 1 of 6+Show Answer & Explanation
Answer: C. 9 × 9
Explanation: The classic puzzle is a 9 × 9 grid split into nine 3 × 3 boxes.
Q2.In Sudoku, each row, column and 3 × 3 box must contain:
Easy- AAny digits, repeats allowed
- BThe digits 1–9 exactly once
- COnly odd digits
- DThe digits 0–9
Sudoku Solving Techniques question 2 of 6+Show Answer & Explanation
Answer: B. The digits 1–9 exactly once
Explanation: The single rule of Sudoku is that 1–9 appear exactly once in every row, column and box.
Q3.A "naked single" refers to:
Moderate- AA cell with only one possible candidate
- BA digit appearing once in the grid
- CAn empty 3 × 3 box
- DA row with one filled cell
Sudoku Solving Techniques question 3 of 6+Show Answer & Explanation
Answer: A. A cell with only one possible candidate
Explanation: When elimination leaves a cell with a single candidate, that value can be entered immediately.
Q4.A "hidden single" occurs when:
Moderate- AA digit is written faintly
- BA digit can go in only one cell within a row, column or box
- CTwo cells share the same candidate
- DA box has one empty cell
Sudoku Solving Techniques question 4 of 6+Show Answer & Explanation
Answer: B. A digit can go in only one cell within a row, column or box
Explanation: Even if that cell has several candidates, the digit has nowhere else to go in the unit, so it must be placed there.
Q5.The proven minimum number of clues for a Sudoku with a unique solution is:
Difficult- A15
- B16
- C17
- D20
Sudoku Solving Techniques question 5 of 6+Show Answer & Explanation
Answer: C. 17
Explanation: An exhaustive computer search published in 2012 established that 17 is the minimum; no 16-clue puzzle has a unique solution.
Q6.How many 3 × 3 boxes are there in a standard Sudoku grid?
Easy- A6
- B8
- C9
- D12
Sudoku Solving Techniques question 6 of 6+Show Answer & Explanation
Answer: C. 9
Explanation: The 9 × 9 grid divides into nine non-overlapping 3 × 3 boxes.
Sudoku Solving Techniques — Frequently Asked Questions
What is the difference between a naked single and a hidden single?+
A naked single is a cell with only one candidate left. A hidden single is a digit that has only one possible cell within a unit, even though that cell may still show several candidates. Hidden singles are easy to miss and are where most beginners stall.
What is the minimum number of clues for a unique Sudoku?+
17. It was proved by exhaustive computer search in 2012 that no valid 16-clue Sudoku with a unique solution exists.
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