Two pipes A and B fill up a half tank in $1.2$ hours. The tank was initially empty. Pipe B was kept open for half the time required by pipe A to fill the tank by itself . Then pipe A was kept open for as much time as was required by the pipe B to fill up $1/3$ of the tank by itself. It was found that the tank was $5/6$ full. The least time in which any of the pipes can the fill the tank fully is ?
Spoiler: The answer is $4$ hours.