Pipes A and B can fill a tank in 4 and 6 hours. Pipe C can empty it in 8 hours. If all pipes are opened together, then the tank will be filled in? Pick one.
A) 3/2hr  B) 24/7 hr  C) 1hr  D) 9/7hr  E) 5/3hr
Net part filled in 1 hour = 1/4 + 1/6 - 1/8 = 7/24   The tank will be full in 24/7 hr   Answer is 24/7 hr
<result>B</result>
Pipes A and B can fill a tank in 4 and 6 hours. Pipe C can empty it in 8 hours. If all pipes are opened together, then the tank will be filled in?