It takes 40 decimeters to enclose a square with iron wire, and enclose a rectangle with the same length of iron wire. Its width is 5 decimeters, what is its area?
用铁丝围一个正方形用去40分米，用同样长的铁丝围一个长方形．它的宽是5分米，面积是多少．
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<gadget id="calculator">15 * 5</gadget>
<output>75</output>

<result>75</result>