The product of the upper base and the height of a trapezoid is 15, and the product of the lower base and the height is 25. What is its area?
一个梯形上底与高的乘积是15，下底与高的乘积是25，它的面积是多少．
<gadget id="calculator">15 + 25</gadget>
<output>40</output>

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

<result>20</result>