person takes a boat from point A to point B downstream, and then goes upstream to point C. It takes 3 hours. It is known that the speed of the boat in still water is 8 kilometers per hour, and the speed of the water flow is 4 kilometers per hour. Kilometers, if the distance between A and C is 12 kilometers, then how many kilometers is the distance between A and B?
某人乘船由A地顺流而下到达B地，然后又逆流而上到达C地，共用了3小时，已知船在静水中的速度为每小时8千米，水流速度为每小时4千米，如果A、C两地间的距离为12千米，那么A、B两地间的距离是多少千米？
<gadget id="calculator">3 + 3</gadget>
<output>6</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<result>18</result>