The speed of a certain boat in still water is 12 kilometers per hour. It takes 12 hours for it to travel from the upstream point A to the downstream point B. The water speed is 3 kilometers per hour. How long does it take to return from point B to point A?
某船在静水中的速度是每小时12千米，它从上游甲地开往下游乙地共花去了12小时，水速每小时3千米，问从乙地返回甲地需要多少小时．
<gadget id="calculator">12 + 3</gadget>
<output>15</output>

<gadget id="calculator">15 * 12</gadget>
<output>180</output>

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<gadget id="calculator">180 / 9</gadget>
<output>20</output>

<result>20</result>