All Covered Cells Are Mines
If a clue still requires exactly as many mines as it has adjacent covered cells, every one of those covered cells must be a mine.
clue − adjacent flags=remaining covered cells
Why the highlighted cell is a mine
- The corner clue is 3.
Its adjacent cells must contain three mines in total.
- Two adjacent mines are already marked.
The clue still requires one mine: 3 − 2 = 1.
- Only one adjacent covered cell remains.
One required mine in one unknown cell forces that cell to be a mine.