There are three piers A, B, and C downstream on a river. The distance between A and B is 20 kilometers, and the distance between B and C is 50 kilometers. A box of goods happened to drop in the water, and the passenger ship was 5 kilometers away from the goods after 10 minutes; when the passenger ship arrived at Wharf A, it found that the goods were missing, and immediately turned around to catch up. When catching up, it happened to meet the cargo ship. It is known that the still water speed of the passenger and cargo ships is the same, so what is the speed of the water flow?
一条河上顺流而下有甲乙丙三个码头，甲、乙距离20千米，乙丙距离50千米；客船和货船分别从乙、丙两个码头同时出发向上游行驶，客船在出发的时候恰好掉了一箱货物在水中，10分钟后客船就距离货物5千米了；当客船到达甲码头的时候发觉货物遗失，立即掉头去追，追上的时候正好遇到货船．已知客、货两船的静水速度相同，那么水流的速度为多少？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">5 / (1/6)</gadget>
<output>30</output>

<gadget id="calculator">50 * 30</gadget>
<output>1_500</output>

<gadget id="calculator">30 * 20</gadget>
<output>600</output>

<gadget id="calculator">1_500 - 600 - 600</gadget>
<output>300</output>

<gadget id="calculator">300 / 50</gadget>
<output>6</output>

<result>6</result>