Make a cylindrical ventilation pipe (hollow) with a diameter of 50cm and a height of 3cm with an iron sheet, and the joint of the iron sheet is 3cm. To make 4 such ventilation pipes, how many square meters of iron sheets are needed at least?
用铁皮做一根直径是50cm、高是3cm的圆柱形通风管(中空)，铁皮接口处为3cm．做4根这样的通风管，至少需要铁皮多少平方米？
<gadget id="calculator">3.14 * 50 * 3</gadget>
<output>471</output>

<gadget id="calculator">50 * 3</gadget>
<output>150</output>

<gadget id="calculator">471 + 150</gadget>
<output>621</output>

<gadget id="calculator">4 / 10_000</gadget>
<output>1/2_500 = around 0.0004</output>

<gadget id="calculator">621 * (1/2_500)</gadget>
<output>621/2_500 = around 0.2484</output>

<result>621/2_500 = around 0.2484</result>