ship transports goods back and forth between two ports 216 kilometers apart. It is known that the ship travels 21 kilometers per hour in still water, and the water velocity between the two ports is 3 kilometers per hour. Then, this ship makes one round trip How much time is needed?
某轮船在相距216千米的两个港口间往返运送货物，已知轮船在静水中每小时行21千米，两个港口间的水流速度是每小时3千米，那么，这只轮船往返一次需要多少时间？
<gadget id="calculator">21 + 3</gadget>
<output>24</output>

<gadget id="calculator">216 / 24</gadget>
<output>9</output>

<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<gadget id="calculator">216 / 18</gadget>
<output>12</output>

<gadget id="calculator">9 + 12</gadget>
<output>21</output>

<result>21</result>