The distance between the two piers is 108 kilometers. It takes 4 hours for a ship to travel along the current and 12 hours to travel against the current. What is the speed of the ship in still water?
两码头相距108千米，1艘轮船顺水行完全程需4小时，逆水行完全程需12小时，这艘轮船在静水中的速度是多少？
<gadget id="calculator">108 / 4</gadget>
<output>27</output>

<gadget id="calculator">108 / 12</gadget>
<output>9</output>

<gadget id="calculator">27 + 9</gadget>
<output>36</output>

<gadget id="calculator">36 / 2</gadget>
<output>18</output>

<result>18</result>