5
4
7
8
1
7
8
4
9
9
1
2
7
6
5
7
6
4
8
9
3
9
1
7
8
2
6
3
1
5
7
9
2
7
5
1
"Chess demands total concentration." — Bobby Fischer
"Chess demands total concentration." — Bobby Fischer
When a box has only two empty cells, you know exactly which two digits fill them.
When a box has only two empty cells, you know exactly which two digits fill them.