Port A and port B are 247.5 kilometers apart. It takes 4.5 hours for a ship to sail from port A to port B. It takes 1 hour longer to return because of headwind than when going back. What is the speed of the water flow?
甲乙两港相距247.5千米，一艘轮船从甲港驶向乙港用了4.5小时，返回时因为逆水比去时多用1小时，则水流速度为多少．
<gadget id="calculator">247.5 / 4.5</gadget>
<output>55</output>

<gadget id="calculator">4.5 + 1</gadget>
<output>5.5</output>

<gadget id="calculator">247.5 / 5.5</gadget>
<output>45</output>

<gadget id="calculator">55 - 45</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<result>5</result>