Looking at specific cells in the puzzle, there are some cells where
only a single number can go.
6
8
2
1
4
3
5
5
6
1
8
6
7
4
7
9
1
6
3
4
5
7
4
2
6
5
8
2
9
7
Example 1: Single Numbers
This is the most fundamental rule of Sudoku. The cell highlighted in dark red in
[r3,c6] can only have the number 9 in it, as there is:
A number 1 in the same 3x3 block, row and column
A number 2 in the same row
A number 3 in the same 3x3 block
A number 4 in the same 3x3 block
A number 5 in the same 3x3 block
A number 6 in the same column
A number 7 in the same column
A number 8 in the same column
Therefore, the only possible value that can go in that cell is a 9. The other
highlighted cells in the example also only have one possible number in them. See
if you can work out which ones.
