A square and a rectangle have the same perimeter. The length of the rectangle is 20 cm and the width is 10 cm. What is the side length of the square in cm?
一个正方形和一个长方形的周长相等，已知长方形的长是20厘米，宽是10厘米，正方形的边长是多少厘米？
<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 * (1/2)</gadget>
<output>15</output>

<result>15</result>