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Each row and each column must contain the numbers 1 through 4 (easy) or 1 through 6 (challenging) without repeating. The numbers within the outlined boxes, called cages, must combine using one of four mathematic functions (addition, subtraction, multiplication and division) to produce the target numbers in the top-left corners. Freebies: Fill in single-box cages with the number in the top-left corner.