The sum of the upper and lower bases of a trapezoid is 18 decimeters, which is three times the height. What is its area?
一个梯形的上、下底之和是18分米，是高的3倍，它的面积是多少．
<gadget id="calculator">18 / 3</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">18 * 3</gadget>
<output>54</output>

<result>54</result>