Two piers on a river are 240 kilometers apart. A boat travels 30 kilometers per hour when sailing along the current and 20 kilometers per hour when sailing against the current. The boat travels back and forth between the two piers three times. What is the average speed of the boat in kilometers per hour.
一条河上的两个码头相距240千米，一条船顺水航行每小时行30千米，逆水航行每小时行20千米，这艘船在两个码头之间往返了3次．船的平均速度是每小时多少千米．
<gadget id="calculator">240 * 2</gadget>
<output>480</output>

<gadget id="calculator">240 / 30</gadget>
<output>8</output>

<gadget id="calculator">240 / 20</gadget>
<output>12</output>

<gadget id="calculator">8 + 12</gadget>
<output>20</output>

<gadget id="calculator">480 / 20</gadget>
<output>24</output>

<result>24</result>