Three trucks of goods arrive at the warehouse gate at the same time. It takes 20 minutes to unload the goods in car A, 10 minutes to unload the goods in car B, and 30 minutes to unload the goods in car C. Only one car can be unloaded. How to arrange the order so that three cars can be unloaded? The sum of truck waiting times is the least?
三车货物同时到达仓库门口，A车货物卸完需要20分钟，B车货物卸完需要10分钟，C货物卸完需要30分钟，只能一车一车卸货，怎么安排顺序，才能使三辆货车等候时间的总和最少？
<gadget id="calculator">10 * 3</gadget>
<output>30</output>

<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">30 + 40 + 30</gadget>
<output>100</output>

<result>100</result>