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Fill in each square with a digit from 1 to 9. The sum of the digits in each row or column will equal the little number given to the left or above it. See the three-square row in the upper left with a 15 to its left? The sum of the digits in those three squares will be 15, and none of the digits will be the same. A row or column ends at a black square, so the three-square row in the upper-middle with a 21 to its left may or may not have digits in common with the 15-row to its left since they’re separated by a black square. Down columns work the same way. Now solve!