A ship is sailing in a river with a water speed of 4 kilometers. It takes 4 hours to travel from A to B and 6 hours to return from B. What is the average speed of the ship in kilometers per round trip? hour.
一只轮船在水速为4千米的河道中航行，从A地顺流到B地用了4小时，从B地返回时用了6小时，这只轮船往返的平均速度是多少千米/时．
<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">4 * 6</gadget>
<output>24</output>

<gadget id="calculator">16 + 24</gadget>
<output>40</output>

<gadget id="calculator">6 - 4</gadget>
<output>2</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 + 4</gadget>
<output>24</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<gadget id="calculator">2 / 10</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">24 * 4 * (1/5)</gadget>
<output>96/5 = around 19.2</output>

<result>96/5 = around 19.2</result>