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Rules of Compoundoku:

  • Solve both left and right Sudokus.
  • In addition, the board below them is the Compound Board of both Sudokus.
  • Each number on the Compound Board should tell either: (1) the number on the left Sudoku, or (2) the sum of both numbers on the left and right Sudokus; in the respective position.

2 Answers 2


I think this is the answer

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Partial Reasoning

For starters the 1 must appear on the left and the 6 must be the sum of both.
Once we make a choice of how to split the 6, the rest of the grid is filled in very quickly (4x4 Sudokus are very restrictive). If we put 3 on the left and 3 on the right or 2 on the left and 4 on the right, we can quickly reach a contradiction. However, putting 4 on the left and 2 on the right allows us to complete the grid in a unique way using the logic. A key step is realising that splitting the bottom 4 will eventually produce a contradiction so it must be that a 4 appears in that position on the left.

  • $\begingroup$ This is correct, nicely done! $\endgroup$
    – athin
    Commented Jan 13, 2020 at 22:54

I got the same answer as hexomino except the right sudoku is

3 1 2 4
4 2 1 3
1 4 3 2
2 3 4 1

so I guess there are at least 2 answers.

I like this puzzle idea though!

  • 2
    $\begingroup$ This grid fails in the left-most square of the second row. Note that athin's rules say it is either the number of the left sudoku, or the sum, but not the number of the right sudoku. $\endgroup$
    – ZanyG
    Commented Jan 14, 2020 at 1:48
  • $\begingroup$ Yep, unfortunately it doesn't tell the number on the right sudoku. But hey, since this genre is pretty new I may experimenting with allowing the right one later on. Or maybe allowing multiplication also, hmm. Yeah let's see lol. $\endgroup$
    – athin
    Commented Jan 14, 2020 at 7:17
  • $\begingroup$ oh yeah i misread the instructions $\endgroup$
    – Timber Lin
    Commented Jan 20, 2020 at 0:19

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