Classic Sudoku · 20
Classic Sudoku · 20: a 9×9 Sudoku with 36 clues and one solution. Play online, use pencil notes and hints, or compare the complete answer.
Use rows and columns: R1C1 is row 1, column 1. Enter digits inside the squares.
Read the position, one decision at a time
Follow the deductions in order. Each step uses the grid after the preceding entry.
Reveal step-by-step explanations
Deduction 1: R1C8 = 8
After the preceding deductions, row 1, column 8 has only candidate 8. Its row, column and box exclude every other digit. Enter 8, then update the remaining candidates; this conclusion does not require consulting the completed answer.
Check the existing digits: row 1: 2, 4, 6, 7; column 8: 1, 3, 4, 5, 9; box: 3, 5, 6, 7. Each digit already present rules out that candidate in this cell.
Deduction 2: R1C9 = 1
After the preceding deductions, row 1, column 9 has only candidate 1. Its row, column and box exclude every other digit. Enter 1, then update the remaining candidates; this conclusion does not require consulting the completed answer.
Check the existing digits: row 1: 2, 4, 6, 7, 8; column 9: 6, 9; box: 3, 5, 6, 7, 8. Each digit already present rules out that candidate in this cell.
Deduction 3: R1C4 = 3
After the preceding deductions, row 1, column 4 has only candidate 3. Its row, column and box exclude every other digit. Enter 3, then update the remaining candidates; this conclusion does not require consulting the completed answer.
Check the existing digits: row 1: 1, 2, 4, 6, 7, 8; column 4: 4, 5, 6, 7, 9; box: 2, 5, 6, 7, 9. Each digit already present rules out that candidate in this cell.
Deduction 4: R1C2 = 5
After the preceding deductions, row 1, column 2 has only candidate 5. Its row, column and box exclude every other digit. Enter 5, then update the remaining candidates; this conclusion does not require consulting the completed answer.
Check the existing digits: row 1: 1, 2, 3, 4, 6, 7, 8; column 2: 1, 8, 9; box: 4, 7, 8. Each digit already present rules out that candidate in this cell.
Understand this position
9×9 grid · 3×3 boxes · 36 clues · 45 empty cells
Start by inspecting row 1, column 8. From the starting grid, its legal candidates are 8. Check the filled digits in that row, column and box before entering a number. Only one candidate remains: this is a naked single.
Read the complete answer by row
9 · 5 · 4 · 3 · 2 · 6 · 7 · 8 · 17 · 8 · 1 · 5 · 9 · 4 · 2 · 3 · 62 · 3 · 6 · 8 · 7 · 1 · 9 · 5 · 41 · 2 · 8 · 7 · 4 · 5 · 6 · 9 · 34 · 7 · 5 · 9 · 6 · 3 · 1 · 2 · 86 · 9 · 3 · 2 · 1 · 8 · 4 · 7 · 58 · 6 · 2 · 1 · 5 · 7 · 3 · 4 · 95 · 1 · 7 · 4 · 3 · 9 · 8 · 6 · 23 · 4 · 9 · 6 · 8 · 2 · 5 · 1 · 7