The distance between the two piers is 418 kilometers. It takes 11 hours for the motorboat to travel downstream and 19 hours to travel upstream. Find the velocity of the current in this river.
两个码头相距418千米，汽艇顺流而下行完全程需11时，逆流而上行完全程需19时．求这条河的水流速度．
<gadget id="calculator">418 / 11</gadget>
<output>38</output>

<gadget id="calculator">418 / 19</gadget>
<output>22</output>

<gadget id="calculator">38 - 22</gadget>
<output>16</output>

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<result>8</result>