A boat travels between A and B. From point A to point B is downstream, travel 35 kilometers per hour; from B to A is upstream, travel 15 kilometers per hour, the average speed of the round trip is how many kilometers per hour.
一只船往返于A、B两地．从A地到B地是顺流，每小时行35千米；从B到A是逆流，每小时行15千米，往返的平均速度是每小时多少千米．
<gadget id="calculator">1 / 35</gadget>
<output>1/35 = around 0.028571</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/35) + (1/15)</gadget>
<output>2/21 = around 0.095238</output>

<gadget id="calculator">2 / (2/21)</gadget>
<output>21</output>

<result>21</result>