A ship sails between two places 300 kilometers apart. It takes 15 hours to go downstream and 25 hours to go upstream. Find the speed of the ship in still water.
一艘轮船在相距300千米的两地航行，顺流而下用了15小时，逆流而上用了25小时，求轮船在静水中的速度．
<gadget id="calculator">300 / 15</gadget>
<output>20</output>

<gadget id="calculator">300 / 25</gadget>
<output>12</output>

<gadget id="calculator">20 - 12</gadget>
<output>8</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>