by **PaulIQ164** » Tue Sep 06, 2005 10:53 pm

Look in column six. There are six unfilled cells in that column. The two in the central box cannot be 4 or 8 or 9, since those numbers already appear in the box. Similarly, the second cell down the column can't be 4 or 8 or 9, since all those numbers already appear in its row. So, out of the six cells still unfilled in the column, we have found three of them, none of which can be a 4 or an 8 or a 9. If those three can't hold the 4, 8 and 9, then the other three must (since the 4, 8 and 9 have to go somewhere in the column). One of these other three cells the cell at column 6, row 6. You have this cell as a possible place for the 1 to go in row 6. But we've just discovered that this cell has to be 4, 6 or 9. So it cannot be the 1.