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I got stuck in the given sudoku puzzle. I can't find any logic to put a number in the empty block. I am playing classic sudoku Android puzzle app.

Maybe it's a XY wing or empty rectangle or any other logic.

  • 7
    $\begingroup$ 5 in column 9 is placed. $\endgroup$
    – Bass
    Nov 5 '21 at 16:20
  • 2
    $\begingroup$ This makes a lot more sense than the answers. In column 9, there must always be one 4 and one 9 no matter the order, so r4c9 must be a 5. $\endgroup$
    – Toby Mak
    Nov 6 '21 at 0:55

Here's how to proceed

Notice that in column 8 (c8), the 5 must be in the lower box (r7 or r9) which means that r5c8 cannot contain a 5 and so r5c3 must contain 5. This in turn implies that the highlighted box must contain 4.
Also notice in column 9 that there is only one possibility for 5 (in row 4).
You should be able to progress from here.

  • $\begingroup$ Why is it that in c8, the 5 must be in the lower box? EDIT: Ah, because the lower box needs to have 5, and is only possible there. Ok. $\endgroup$
    – justhalf
    Nov 6 '21 at 6:57

A 5 or 9 at row 8, column 5 implies a non-unique solution, so 8,5 is 4. To get to the highlighted square, 4,7 becomes 9 and 7,8 cannot be 9. This makes a non-unique corner that forces 4,3 to be 9. 5,3 becomes 5 and we find a 4 at 3,7.

In addition, column 9 has a 4,9 group, so 9,4 is a 5. This forces 4,1 to 4, 9,1 to 5 and 7,3 to 4.

Of course, the answer from hexomino is also correct, I only add these as they are slightly more direct.


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