The upper base of a trapezoid is 5 cm and the lower base is 7 cm. If the lower base is extended by 2 cm, the area of the trapezoid increases by 4 cm2. What is the area of the original trapezoid in cm2?
一个梯形的上底是5厘米，下底是7厘米，如果把下底延长2厘米，则梯形的面积增加4平方厘米，原梯形面积为多少平方厘米．
<gadget id="calculator">5 + 7</gadget>
<output>12</output>

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

<gadget id="calculator">12 * 4 * (1/2)</gadget>
<output>24</output>

<result>24</result>