The speed of a steamship in still water is 21 kilometers per hour. The boat sailed upstream from City A for 8 hours and arrived at City B, which is 144 kilometers away. How many hours does it take for the ship to return from city B to city A?
一只轮船在静水中的速度是每小时21千米，船从甲城开出逆水航行了8小时，到达相距144千米的乙城．这只轮船从乙城返回甲城需多少小时？
<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>