A cylindrical water pipe with an inner diameter of 20 cm and a flow rate of water in the pipe is 40 cm per second, how many liters of water flow through it per second?
一根圆柱形水管，内直径为20厘米，水在管内的流速为每秒40厘米，每秒流过的水是多少升？
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 ** 2</gadget>
<output>100</output>

<gadget id="calculator">40 / 1_000</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">3.14 * 100 * (1/25)</gadget>
<output>12.56</output>

<result>12.56</result>