dailysudoku.com

[wood2wn]

When I fill in the blank grid I get a hint, but I can't figure out the logic.

9 - - 6 - - - 5 8
8 - 4 - - - 6 1 -
6 2 - - 8 - - 3 -
2 - - 7 - 3 5 - 4
7 - 6 - 5 - 1 - 3
3 - - 1 - 8 - - -
4 7 8 3 1 6 9 2 5
5 6 2 8 - - 3 7 1
1 9 3 5 2 7 8 4 6

The tip I get is is 7 in first row just before 6 , and I can´t figure out why.
Can anyone please tell me why?

[David Bryant]

Hi, wood2wn! Welcome to the forum.

The answer you seek lies in row 3.

I think it's fairly obvious that the only possibilities for r1c3 are {1, 7}. It's also clear that the only way a "7" can fit in the top left 3x3 box is either at r1c3 or at r3c3.

Let's analyze row 3. The only possibilities at r3c4 are {4, 9}. The only possibilities at r3c7 are {4, 7}. The only possibilities at r3c9 are {7, 9}. Therefore the values {4, 7, 9} must occupy the cells r3c4, r3c7, & r3c9 in some order ("naked triplet"), leaving the values {1, 5} to occupy the two cells r3c3 & r3c6.

Therefore the "7" must go in r1c3, because it can't be placed at r3c3. dcb