There is a vegetable field in the shape of a trapezoid, with an upper base of 12 meters, a lower base of 8 meters, and a height of 30 meters. Last year, a total of 12,000 kilograms of cabbage was harvested. How many kilograms of cabbage were harvested on average per square meter?
有一块菜地为梯形，上底是12米，下底8米，高是30米，去年共收白菜12000千克，平均每平方米收白菜多少千克？
<gadget id="calculator">12 + 8</gadget>
<output>20</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">20 * 15</gadget>
<output>300</output>

<gadget id="calculator">12_000 / 300</gadget>
<output>40</output>

<result>40</result>