It takes 8 hours for a boat to travel 320 kilometers along the current, and the speed of the current is 15 kilometers per hour. How many hours does it take for this boat to travel upstream?
一只船顺水行320千米需用8小时，水流速度为每小时15千米，这只船逆水行这段路程需用几小时？
<gadget id="calculator">320 / 8</gadget>
<output>40</output>

<gadget id="calculator">40 - 15 - 15</gadget>
<output>10</output>

<gadget id="calculator">320 / 10</gadget>
<output>32</output>

<result>32</result>