There is a river between two cities. A ship sails between the two cities. It takes 6 hours to sail forward and 8 hours to go upstream. The speed of the current is 2.5 kilometers per hour. Find the speed of the boat in still water.
两个城市间有一条河，一艘轮船在两个城市间航行，顺流需要6小时，逆流要8小时，水流速度为每小时2.5千米，求船在静水中的速度．
<gadget id="calculator">2.5 * 6</gadget>
<output>15</output>

<gadget id="calculator">2.5 * 8</gadget>
<output>20</output>

<gadget id="calculator">15 + 20</gadget>
<output>35</output>

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

<gadget id="calculator">35 / 2</gadget>
<output>35/2 = around 17.5</output>

<result>35/2 = around 17.5</result>