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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 to the left or above it. See the five-box row in the upper left with a 30 to its left? The sum of the digits in those five boxes will be 30, and none of the five digits will be the same. A row or column ends at a black square, so the four-box row in the upper-right with a 28 to its left may or may not have digits in common with the 30-row to its left since they’re separated by a black square. Down columns work the same way. Now solve!