The height of a trapezoid is 4 cm, the upper base is 8 cm, and the lower base is 7 cm. What is the area of the trapezoid?
梯形的高是4厘米，上底8厘米，下底7厘米，这个梯形的面积是多少．
<gadget id="calculator">7 + 8</gadget>
<output>15</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<result>30</result>