The barrage of a hydropower station has a trapezoidal cross-section, its lower base is 18 meters, the upper base is half of the lower base, and its height is 20 meters. What is the area of this cross section in square meters?
有一座水电站的拦河坝的横截面是梯形，它的下底是18米，上底是下底的一半，高是20米．这个横截面的面积是多少平方米？
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">9 + 18</gadget>
<output>27</output>

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

<gadget id="calculator">27 * 10</gadget>
<output>270</output>

<result>270</result>