A trapezoidal billboard, the sum of its upper and lower bases is 30 decimeters, and its height is 12 decimeters, what is its area in square decimeters? ．
一块梯形广告牌，它的上底与下底的和是30分米，高12分米，它的面积是多少平方分米．．
<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">30 * 6</gadget>
<output>180</output>

<result>180</result>