The speed of a certain boat in still water is 15 kilometers per hour. It took 8 hours for it to sail from the upstream point A to the downstream point B, and the water speed was 3 kilometers per hour. How many hours does it take for the boat to return from point B to point A?
某船在静水中的速度是每小时15千米，它从上游甲地开往下游乙地共花了8小时，水速每小时3千米．则这船从乙地返回甲地需要几小时？
<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">8 / 12</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">18 * (2/3)</gadget>
<output>12</output>

<result>12</result>