There is a trapezoid with an upper base of 5 cm and a lower base of 7 cm. If only the upper base is increased by 3 cm, the area will increase by 4.5 cm2. So what is the area of the original trapezoid in square centimetres?
有一个梯形，它的上底是5厘米，下底7厘米，如果只把上底增加3厘米，那么面积就增加4.5平方厘米．那么原来梯形的面积是多少平方厘米．
<gadget id="calculator">5 + 7</gadget>
<output>12</output>

<gadget id="calculator">2 / 3 / 2</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">12 * 4.5 * (1/3)</gadget>
<output>18</output>

<result>18</result>