A 2.5-meter-long ventilation pipe has a cross-section of a square with side lengths of 0.5 meters. How many square meters of iron sheets are needed if iron sheets are used to make 50 knots of such ventilating pipes?
一节2.5米长的通风管，横截面是边长为0.5米的正方形．如果用铁皮做这样的通风管50节，需要多少平方米的铁皮？
<gadget id="calculator">0.5 * 4 * 2.5 * 50</gadget>
<output>250</output>

<result>250</result>