It is known that wharf A is upstream of B, and a ship departs from A and goes back and forth between A and B non-stop (the time of turning around is not counted). It takes 12 hours to return to Pier A three times. Q: How many hours does it take for the boat to travel from port B to port A?
已知码头A在B的上游，一艘船从A出发不停的在A，B间往返(掉头的时间不计)，若船从出发到第二次到达码头B用5.5小时，从出发到第3次返回码头A用12小时．问：船从码头B行驶到A需要几小时？
<gadget id="calculator">4 - 1.5</gadget>
<output>2.5</output>

<result>2.5</result>