The average of the upper and lower bases of a trapezoid is 40 cm, and the height is 12 cm. What is its area?
一个梯形上、下底的平均数为40厘米，高12厘米，它的面积是多少．
<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">40 * 2 * 6</gadget>
<output>480</output>

<result>480</result>