The upper base of the trapezoid is 4 cm, the lower base is twice the upper base, and the height is 3 cm. What is the area of the trapezoid?
梯形的上底是4厘米，下底是上底的2倍，高是3厘米，梯形的面积是多少．
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">4 + 8</gadget>
<output>12</output>

<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">12 * (3/2)</gadget>
<output>18</output>

<result>18</result>