swimmer goes upstream in the river and loses his water bottle under bridge A, which is washed away by the water. After continuing to swim for 20 minutes, he finds that the water bottle is lost, so he immediately returns to search for the water bottle, and pursues it under bridge B, which is 2 kilometers downstream from bridge A. When it reaches the kettle, what is the speed of the river in kilometers per hour?
游泳者在河中逆流而上，于桥A下面将水壶遗失被水冲走，继续前游20分钟后他发现水壶遗失，于是立即返回追寻水壶，在桥A下游距桥2公里的桥B下面追到了水壶，那么该河水流的速度是每小时多少公里．
<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>