Answer: 10cm Step-by-step explanation Perimeter of the rectangle = 2(16+4) = 40cm Assume the length of a side of the square as x. Therefore the perimeter of ssquare can be wwritten as 4x.(all sides are equal in a square ) And we can build up a simple equation as 4x=40 When u simplify this equation,you get the x value as 10cm. That's the side length of the square.