Xiao Ming was rowing upstream in the river, and accidentally dropped the kettle into the river. After 20 minutes, Xiao Ming found it. He immediately went back to search, and finally found the kettle 600 meters downstream from the lost place. So what is the speed of the water flow?
小明在河中划船逆流而上，不慎将水壶掉进河中，经过20分钟小明才发现，他立即返回寻找，结果在离丢失地点下游600米处找到水壶．那么水流的速度是多少．
<gadget id="calculator">20 + 20</gadget>
<output>40</output>

<gadget id="calculator">600 / 40</gadget>
<output>15</output>

<result>15</result>