The sum of the upper and lower bases of a trapezoid is 20cm, and the height is 8cm. What is the area of the trapezoid?
一个梯形上底和下底的和是20cm，高是8cm，这个梯形的面积是多少．
<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<result>80</result>