Column 6 has a total of 18, but we already have some cells filled in for this column. With the cells
remaining there is only one way to reach a total, and that is 1 + 4 + 6 + 7, i.e. cells A and B
must be filled.
The bottom row has a total of 24, and we already have 6 + 7 = 13. This leaves us needing 24-13=11 from
the remaining cells, and there are only a few ways we can do this,
- 1 + 4 + 5 = 11
- 2 + 3 + 5 = 11
Both of these ways include the number '5', i.e. cell C must be filled.
We can also look at the third column, with a total of 9. There are many ways to reach a total of 9 for
this column, but since we can't use the '2' for this column, none of them include '7', i.e. cell D must