A ship sails between two piers. It takes 4 hours to sail along the current and 5 hours to sail against the current. The speed of the water is 2 kilometers. Find the speed of the ship in still water.
一条轮船在两码头间航行，顺水航行需4小时，逆水航行需5小时，水速是2千米，求这轮船在静水中的速度．
<gadget id="calculator">2 + 2</gadget>
<output>4</output>

<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">16 + 2</gadget>
<output>18</output>

<result>18</result>