The distance between A and B is 144 kilometers. The steamboat sails upstream from B to A for 8 hours. It is also known that the steamboat travels 21 kilometers per hour in still water. How many hours does it take for the steamer to sail back to B?
甲、乙两个码头相距144千米，汽船从乙码头逆水行驶8小时到达甲码头，又知汽船在静水中每小时行21千米，那么汽船顺流开回乙码头需要多少小时．
<gadget id="calculator">144 / 8</gadget>
<output>18</output>

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

<gadget id="calculator">21 + 3</gadget>
<output>24</output>

<gadget id="calculator">144 / 24</gadget>
<output>6</output>

<result>6</result>