PIKO · LEARN & EXPLAIN
4×4 Sudoku: explain every placement
Use a row, a column and a small box together to rule out candidates. A completed row alone does not prove the board is correct.
Before you start
Start when you can recognise 1, 2, 3 and 4. This example is untimed; a pencil and paper are enough.
Work through this example
A dot means an empty cell. Keep every given number. Each row, column and outlined 2×2 box must contain 1–4 exactly once. Rows are numbered from the top and columns from the left.
| 1 | · | 3 | · |
| · | 4 | · | 2 |
| 2 | · | 4 | · |
| · | 3 | · | 1 |
Reason step by step
- Row 1 lacks 2 and 4. Column 2 already has 4, so row 1 column 2 must be 2. The last cell in row 1 is 4.
- Row 2 lacks 1 and 3. Column 1 already has 1, so row 2 column 1 is 3 and row 2 column 3 is 1. Check both top boxes.
- Row 3 lacks 1 and 3. Column 2 already has 3, so row 3 column 2 is 1 and its last cell is 3. Column 1 then needs 4; row 4 column 3 needs 2.
Completed board
| 1 | 2 | 3 | 4 |
| 3 | 4 | 1 | 2 |
| 2 | 1 | 4 | 3 |
| 4 | 3 | 2 | 1 |
A mistake worth understanding
Writing 4 in row 1 column 2 fills a missing row number but repeats 4 in column 2 and the top-left box. Check all three constraints before keeping a number.
Try it yourself
Cover the completed board. What belongs in row 4 column 1? Name two clues that support it.
Show the answer and reasoning
4. Column 1 has 1, 3 and 2; the bottom-left box has 2, 1 and 3. Both need 4.
Use the idea in the game
In the game, choose the first difficulty for 4×4. The second uses 6×6 with 2×3 boxes and numbers 1–6. Apply the same checks; do not reuse this answer pattern.
Open the activity →About & help
These examples were written for Piko Game. Generated game questions may differ. A score describes a game attempt, not mastery of a school curriculum.
Found an error? Send the page address and the step that needs checking to j565718319@gmail.com。
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