A small boat with a speed of 20 km/h in still water travels 391 km upstream for 23 hours. How many hours will it take to return it to its original place?
一只小船在静水中速度为20千米/小时，它在391千米的逆水途中行了23小时．求它返回原处需用几个小时？
<gadget id="calculator">391 / 23</gadget>
<output>17</output>

<gadget id="calculator">20 - 17</gadget>
<output>3</output>

<gadget id="calculator">20 + 3</gadget>
<output>23</output>

<result>17</result>