The outer diameter of the train wheel is 0.9 meters long. If it rotates 400 times per minute, how many kilometers does the train advance per hour?
火车轮的外直径长0.9米，如果它每分钟转400周，那么这列火车每小时前进多少千米？
<gadget id="calculator">400 / 1_000</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">3.14 * 0.9 * (2/5) * 1 * 60</gadget>
<output>67.824</output>

<result>67.824</result>