If the sum of the upper and lower bases of a trapezoid is 20 cm and the height is 4 cm, what is the area of the trapezoid in square centimeters?
已知一个梯形的上、下两底的和20厘米，高是4厘米，则这个梯形的面积是多少平方厘米．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<result>40</result>