There is a distance of 240 kilometers between the upstream and downstream ports of a large river. A ship travels downstream at a speed of 30 kilometers per hour and upstream at a speed of 20 kilometers per hour. So what is the average speed of the ship going back and forth between the two docks in kilometers per hour?
一条大河上游与下游的两个码头相距240千米，一艘航船顺流而下的速度为每小时航行30千米，逆流而上的速度为每小时航行20千米．那么这艘船在两码头之间往返一次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>