A person goes down the river on a raft. When he reaches a bridge, he wants to exercise, so he jumps into the water and swims along the river. After 10 minutes, he turns around and swims back, and meets the raft 500 meters away from the bridge. Assuming that the speed of the water flow and the speed of a person swimming in still water remain constant, find the speed of the water flow.
一人乘木筏在河面顺流而下，行至一座桥下时此人想锻炼一下身体，便跳入水中顺水游泳，10分钟后掉头往回游，在离桥500米远的地方与木筏汇合，假设水流速度及比人在静水中游泳的速度一直不变，求水流速度．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">500 / (1/3)</gadget>
<output>1_500</output>

<result>1_500</result>