The sum of the upper and lower bases of a trapezoid is 15 meters and the height is 4 meters. What is the area in square meters?
一个梯形上底与下底的和是15米，高是4米，面积是多少平方米．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

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

<result>30</result>