A circle is surrounded by an iron wire, and its diameter is exactly 20 decimeters. If this iron wire is changed into a square, what is the length of its side?
用一根铁丝围成一个圆，直径正好是20分米，如果把这根铁丝改围成一个正方形，它的边长是多少分米．
<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">3.14 * 5</gadget>
<output>15.7</output>

<result>15.7</result>