Pipes A and B can fill a tank in 2 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in?  Choose the correct choice from the following.
A) 3 9/97 hrs  B) 3 9/77 hrs  C) 3 9/17 hrs  D) 1 5/7 hrs  E) 3 9/19 hrs
Net part filled in 1 hour = 1/2 + 1/6 - 1/12   = 7/12   The tank will be full in 12/7 hrs, i.e., 1 5/7 hrs.   Answer:1 5/7 hrs
<result>D</result>
Pipes A and B can fill a tank in 2 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in?