A man was swimming in the river and went upstream. When he reached a bridge, he lost his water bottle. He swam forward for another 20 minutes before realizing it. He immediately turned back and chased after the water bottle. He lost the water bottle when he was 2 kilometers away from the bridge. To catch up, how many kilometers does the river flow per hour?
某人在河中游泳，逆流而上，当游到一桥下时，将水壶丢失，他又向前游了20分钟才发觉，立即折回追赶水壶，当追到离桥2千米处才将水壶追上，求河水每小时流多少千米？
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">2 / (2/3)</gadget>
<output>3</output>

<result>3</result>