A ship sails from port A to port B. It is known that the speed of the ship in still water is 15 kilometers per hour, and the speed of the current is 3 kilometers per hour. It travels along the current for 6 hours when it goes there, and it goes against the current when it returns. How many hours did you travel when you returned?
一艘轮船从甲港开往乙港，已知船在静水中的速度是每小时15千米，水流的速度是每小时3千米，去时顺水行了6小时，返回时是逆水，问返回时行了几小时？
<gadget id="calculator">15 + 3</gadget>
<output>18</output>

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

<gadget id="calculator">18 / 12</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">6 * (3/2)</gadget>
<output>9</output>

<result>9</result>