A dam is 80 meters long, the cross section is a trapezoid, the upper base is 8 meters long, the lower base is 13 meters long, and the height is 5 meters. How many cubic meters of earth and rock are needed to build this dam?
一堤坝长80米，横截面是一个梯形，上底长8米，下底长13米，高5米，修筑这个堤坝需要多少立方米土石？
<gadget id="calculator">8 + 13</gadget>
<output>21</output>

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

<gadget id="calculator">21 * (5/2) * 80</gadget>
<output>4_200</output>

<result>4_200</result>