Chapter 2. Complete rows, columns and 3x3 clusters
For completing an item, it is important that only a few numbers are unknown  maximum is about three. . Complete by using excluding methods.
The sudoku puzzle on the left shows three empty cells in the first row, signed with blue, green and yellow question marks. To find out missing numbers we have to fix which one are still missed in the first line  these are ‘4’, ‘7’ and ‘8’. After that let’s have a look at the left (blue) field in the lefttop 3x3 cluster. In this cell the residual numbers ‘4’ and ‘8’ could be placed, but if you look on the both adjacent 3x3 clusters, there is already an ‘8’ in each. According to our rules, the result is that the 8 has to be filled in our blue cell.
. Now, there only remain ‘4’ and ‘7’. As you can see on the right picture, further solution is quite easy. In the column of our second (green) question mark there is already a ‘7’, that means you are only allowed to fill in no. ‘4’. At last there only one number for one left cell in our sudoku puzzle. The yellow number is 7. .
