A power boat and a raft both left dock $A$ on a river and headed downstream. The raft drifted at the speed of the river current. The power boat maintained a constant speed with respect to the river, which was three times the speed of the river current. The power boat reached dock $B$ downriver, then immediately turned and traveled back upriver. It eventually met the raft on the river 15 hours after leaving dock $A$. How many hours did it take the power boat to go from $A$ to $B$?