A trapezoid, the sum of the upper and lower bases is 20 cm, and the height is 6 cm, what is its area?
一个梯形，上底与下底的和是20cm，高是6厘米，它的面积是多少．
<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<result>60</result>