Uncle Wang wants to cover a rectangular vegetable field with film. The length of this field is 40 meters and the width is half of the length. How many square meters of film does Uncle Wang need to prepare at least?
王伯伯要为一块长方形的菜地盖上薄膜，这块地的长是40米，宽是长的一半，王伯伯至少要准备多少平方米的薄膜？
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">40 * 20</gadget>
<output>800</output>

<result>800</result>