The distance between A and B ports is 200 kilometers. Ships A and B sail from port A to port B at the same time; ship A travels 45 kilometers per hour and ship B travels 35 kilometers per hour in still water; ship A arrives at port B immediately. Return and meet B face to face after another 0.5 hour. What is the velocity of the water in kilometers per hour.
A、B两港相距200千米，甲乙两船同时从A港顺流而下去B港；静水中甲船每小时行45千米，乙船每小时行35千米；甲船到B港立即返回，又过0.5小时与乙迎面相遇．水流速度为每小时多少千米．
<gadget id="calculator">45 + 35</gadget>
<output>80</output>

<gadget id="calculator">0.5 * 80</gadget>
<output>40</output>

<gadget id="calculator">200 - 40</gadget>
<output>160</output>

<gadget id="calculator">200 / 160</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">(5/4) * 35</gadget>
<output>175/4 = around 43.75</output>

<gadget id="calculator">45 - (175/4)</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">(5/4) - 1</gadget>
<output>1/4 = around 0.25</output>

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

<result>5</result>