A new barrage is built, its upper base is 4 meters, which is twice the lower base, and its height is 2.5 meters. What is its cross-sectional area in square meters?
新筑一个拦河坝，它的上底是4米，是下底的2倍，高是2.5米．它的横截面积是多少平方米？
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">4 + 2</gadget>
<output>6</output>

<gadget id="calculator">2.5 / 2</gadget>
<output>1.25</output>

<gadget id="calculator">6 * 1.25</gadget>
<output>7.5</output>

<result>7.5</result>