In a trapezoidal field with an upper base of 42 meters, a lower base of 68 meters, and a height of 20 meters, draw out the largest parallelogram to plant cotton, and the rest to grow vegetables. How big is the place to grow vegetables?
在一块上底是42米，下底是68米，高是20米的梯形地里划出一块最大的平行四边形地种棉花，其余种青菜．种青菜的地方有多大？
<gadget id="calculator">68 - 42</gadget>
<output>26</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">26 * 10</gadget>
<output>260</output>

<result>260</result>