2
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Take the following Killer Sudoku (from the app "Killer Sudoku" by Studio Goya LLC) I have been struggling with for the last few hours. "Penciled" numbers are my notes about which numbers are possible in the cells.

Killer Sudoku Puzzle

I cannot seem to find the next step. The app I am using says that R1c3 can only be a 5. However, I fail to see the reason. Anything I have tried still allows a 4 to be placed there. What is the (probably very obvious) thing that I am missing here?

For those not familiar with Killer Sudoku rules: Normal Sudoku rules apply. In addition, numbers in cages must sum to the small number in the top left corner of the cages. Numbers cannot be repeated in cages.

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6
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Assuming you are asking about R1C3,

If you look at the top-left 3x3 box, there is only one square where the 5 can go. You've already correctly ruled out 5 from the cages summing to 6 and 7.

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    $\begingroup$ As I suspected, the answer was so obvious that I missed it. Thank you very much. $\endgroup$
    – jeyk
    Aug 16 at 15:55
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    $\begingroup$ Also, box 1 has a three-cell 6-cage, which you have correctly identified as a forced 1-2-3 triple. Since r3c1 is in the same box, and belongs to a three-cell 7-cage, you could have started the sudoku by putting a 4 in there. $\endgroup$
    – Bass
    Aug 16 at 20:20

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