A vessel has three pipes connected to it two to supply liquid and one to draw liquid. The test alone can fill the vessel in `4 1/2` hours, the second in 3 hours and the third can empty it in `1 1/2` hours. It all the pipes are opened simultaneously when the vessel is hall full, how soon will it be emptied? `4 1/2hou r s` b. `5 1/2\ hou r s` c. `6 1/2\ hou r s` d. none of these