A ship sailed 140 kilometers downstream and 80 kilometers upstream, and it took 15 hours; it also took 15 hours to sail 60 kilometers downstream and 120 kilometers upstream. Find the velocity of the water flow.
一艘轮船顺流航行140千米，逆流航行80千米，共用了15小时；顺流航行60千米，逆流航行120千米，也用了15小时．求水流的速度．
<gadget id="calculator">140 - 60</gadget>
<output>80</output>

<gadget id="calculator">80 * 3</gadget>
<output>240</output>

<gadget id="calculator">240 + 60</gadget>
<output>300</output>

<gadget id="calculator">300 / 15</gadget>
<output>20</output>

<gadget id="calculator">120 - 80</gadget>
<output>40</output>

<gadget id="calculator">60 / 20</gadget>
<output>3</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">3 / 12</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">40 * (1/4)</gadget>
<output>10</output>

<gadget id="calculator">20 - 10</gadget>
<output>10</output>

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

<result>5</result>