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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.