3.1. Five identical squares, standing in a row, were cut by two horizontal lines. The sum of the perimeters of the resulting 15 rectangles is 800 cm. Indicate in centimeters the length of the original squares.

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# Answer: 20

Solution. Let's calculate how many times the side of the original square is repeated in the sum of all perimeters. The sides of the rectangle are counted once (a total of 12), and the crossbars are counted twice ($4 \cdot 2 + 10 \cdot 2 = 28$). In total, $40.800: 40=20$ cm - the side of the square.