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Fill in each square in this grid with a digit from 1 to 9. The sum of the digits in each row or column will equal the little number given just to the left of or above it. See the two-box row in the upper-left of the grid with a 6 to the left of it? The sum of the digits in those two squares will be 6, and they won’t be the same number (i.e., two 3s). A row or column ends at a black square, so the two-square row in the upper-center with a 9 to its left may or may not have digits in common with the 6-row to its left; they’re considered different rows because they’re separated by a black square. Down columns work the same way. It’s psychotic!