The upper base is 4 cm, the lower base is twice the upper base, and the height is 5 cm. What is the area in square centimeters?
上底是4厘米，下底是上底的2倍，高是5厘米的梯形面积是多少平方厘米．
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

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

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">12 * (5/2)</gadget>
<output>30</output>

<result>30</result>