It takes 16 hours for a ship to sail downstream from Pier A to Pier B, and it takes 20 hours for this ship to sail upstream from Pier B to Pier A. It is known that the speed of this ship in still water is 18 kilometers per hour, then the speed of the water flow What is the speed in kilometers per hour.
一艘轮船由A码头顺水航行到B码头需要16小时，这艘轮船由B码头逆水航行到A码头需要20小时，已知这艘轮船在静水中的速度为每小时18千米，则水流的速度为每小时多少千米．
<gadget id="calculator">18 * 20</gadget>
<output>360</output>

<gadget id="calculator">18 * 16</gadget>
<output>288</output>

<gadget id="calculator">360 - 288</gadget>
<output>72</output>

<gadget id="calculator">16 + 20</gadget>
<output>36</output>

<gadget id="calculator">72 / 36</gadget>
<output>2</output>

<result>2</result>