A trapezoid whose lower base is 8 centimeters, if its upper base is increased by 3 centimeters, it becomes a parallelogram, and the area is increased by 15 square centimeters, what is the area of the original trapezoid in square centimeters?
一个梯形，它的下底是8厘米，如果将他的上底增加3厘米，正好变成一个平行四边形，这时面积增加15平方厘米，原来的梯形面积是多少平方厘米．
<gadget id="calculator">8 - 3</gadget>
<output>5</output>

<gadget id="calculator">5 + 8</gadget>
<output>13</output>

<gadget id="calculator">2 / 2 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">13 * 15 * (1/2)</gadget>
<output>195/2 = around 97.5</output>

<result>195/2 = around 97.5</result>