A ship travels at a speed of 28 km/h, arrives at the port in 3 hours, and takes 3.5 hours to return. What is the speed of the water in kilometers per hour?
某船顺水速度28公里/小时，3小时到达港口，返回时用了3.5小时，水流速度是每小时多少公里．
<gadget id="calculator">3 / 3.5</gadget>
<output>0.857143</output>

<gadget id="calculator">28 * 0.857143</gadget>
<output>24.000004</output>

<gadget id="calculator">28 - 24.000004</gadget>
<output>3.999996</output>

<gadget id="calculator">3.999996 / 2</gadget>
<output>1.999998</output>

<result>1.999998</result>