A ventilation duct is 5 decimeters long and has a cross-section of a square with side length 2 centimeters. At least how many square decimeters of plastic are needed to make 10 such ventilation pipes.
一个通风管长5分米，横截面是边长为2厘米的正方形．做10个这样的通风管至少要多少平方分米的塑料．
<gadget id="calculator">2 / 10</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) * 4 * 5 * 10</gadget>
<output>40</output>

<result>40</result>