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 5 kilometers per hour. How many kilometers per hour does the ship travel 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，水速是每小时5千米，这条船在静水中每小时行多少千米．
<gadget id="calculator">5 * 2 * 4</gadget>
<output>40</output>

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">40 / 1</gadget>
<output>40</output>

<gadget id="calculator">5 / 4</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">40 * (5/4)</gadget>
<output>50</output>

<gadget id="calculator">50 + 40</gadget>
<output>90</output>

<gadget id="calculator">90 / 2</gadget>
<output>45</output>

<result>45</result>