To build a road, 1.5 kilometers have been repaired in 3 days. At this speed, the task can be completed in 21 days. How long is this road in kilometers? (Solve with ratio)
修一条公路，开工3天修了1.5千米，照这样的速度，再修21天就可以完成任务，这条公路长多少千米？(用比例解)
<gadget id="calculator">1.5 / 3</gadget>
<output>0.5</output>

<gadget id="calculator">0.5 * 21</gadget>
<output>10.5</output>

<gadget id="calculator">10.5 + 1.5</gadget>
<output>12</output>

<result>12</result>