dailysudoku.com

[Steve M]

This one has stumped both my wife and me. We both got to this point.

4 6 12 8 27 9 3 5 17
9 58 12 1357 2347 124 67 1478 1467
58 7 3 15 46 146 2 148 9
2 9 8 4 5 7 1 6 3
3 4 6 2 1 8 59 79 57
7 1 5 6 9 3 4 2 8
1 25 4 9 267 26 8 3 567
568 258 7 13 23468 1246 59 149 1456
68 3 9 17 4678 5 67 147 2

What is the key here?

[David Bryant]

Hi, Steve!

This puzzle has already been discussed extensively, over here.

Briefly, there are only two ways to enter a "7" in row 1 (at r1c5 & r1c9) and there are also only two ways to enter a "7" in row 7 (at r7c5 & r7c9). So there's either a "7" at r1c5 and a "7" at r7c9, or else there's a "7" at r7c5 and a "7" at r1c9. Either way there cannot possibly be a "7" at r5c9, therefore r5c9 = 5.

Alternatively you can analyze the possible positions for a "7" in columns 4 & 7 to conclude that the "7" in column 8 must appear at r5c8.

Happy sudokuing! dcb

[Steve M]

Thanks , That was what we needed.