# Compoundoku wants REALLY BIG!

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.

Credit to @athin for the original idea of this sudoku variant.
The original and 'BIG' version can be found here and here.

• This probably took longer to create than it will for anyone to solve. If you do attempt to solve it, please let me know how long it took you, to satisfy my curiosity. Aug 12, 2020 at 12:23
• BTW, it took me a bit more than an hour to solve, but it wasn't dedicated time. Aug 12, 2020 at 16:52
• I just wanted to revisit this puzzle today and saw this, what's the chance! XD Thanks for the puzzle! Aug 14, 2020 at 2:39

SOLUTION

REASONING

The first step is to address the smaller and larger numbers in the compound grid first. The compound 1 has to correspond with a 1 in the left grid. The compound 2's in the grid must either be 2's in the left grid, or 1/1 (a 1 in both the left and right grids). Since there is already a 1 in the fourth row/third column of the left grid, this forces the compound 2's in the middle-left and center squares to be 2's in the left grid.

Similarly, the two compound 18's must be 9/9. Now look at the compound 16's in the bottom middle square. Since there are three of them, they must be 7/9, 8/8, and 9/7 in the left/right grids. But there is already a 9 in both right columns, so this one must be 8/8. Using similar logic, the compound 16's in the lower left and lower right squares must also be 8/8. Moving to bottom-center, the compound 12 in this square must be 6/6, since 789 are already obligated in this square both grids. The same logic then forces the compound 10 in this square to be 5/5. At this point, our grid looks like:

• I won't edit since it seems like you're in the middle of editing, but formatting tip: to get a more evenly spaced square of numbers, you can use code formatting (use  at the start and end of each row, or <code> and </code>`) and insert as many spaces as necessary between numbers in a row. It will work inside a spoilertag provided you also use double space at the end of each line. Aug 12, 2020 at 14:57