A cistern has three pipes A, B and C. Pipes A and B are inlet pipes whereas C is an outlet pipe. Pipes A and B can fill the cistern separately in 3…

A cistern has three pipes A, B and C. Pipes A and B are inlet pipes whereas C is an outlet pipe. Pipes A and B can fill the cistern separately in $\displaystyle 3$ hours and $\displaystyle 4$ hours respectively; while pipe C can empty the completely filled cistern in $\displaystyle 1$ hour. If the pipes A, B and C are opened in order at $\displaystyle 5$, $\displaystyle 6$ and $\displaystyle 7$ a.m. respectively, at what time will the cistern be empty?