The average length of the upper and lower bases of a trapezoid is 30 cm, and the height is 2 decimeters. What is the area of the trapezoid?
一个梯形的上底与下底的平均长度是30厘米，高2分米，这个梯形的面积是多少．
<gadget id="calculator">2 * 10</gadget>
<output>20</output>

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

<gadget id="calculator">30 * 2 * 10</gadget>
<output>600</output>

<result>600</result>