Zhang Hong surrounds a square vegetable field with a 40-meter-long rope. What is the area of the vegetable field?
张红用一条长40米的绳子围一块正方形菜地，这块菜地的面积是多少．
<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<gadget id="calculator">10 * 10</gadget>
<output>100</output>

<result>100</result>