A trapezoidal mountain, the upper base is 40 meters long, the lower base is 20 meters longer than the upper base, and the height is half of the upper base. How many hectares is this land?
一块梯形山地，上底长40米，下底比上底长20米，高是上底的一半．这块地有多少公顷？
<gadget id="calculator">40 + 20 + 40</gadget>
<output>100</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 / 2 / 10_000</gadget>
<output>1/1_000 = around 0.001</output>

<gadget id="calculator">100 * (1/1_000)</gadget>
<output>1/10 = around 0.1</output>

<result>1/10 = around 0.1</result>