Pipes A and B can fill a tank in 4 and 7 hours. Pipe C can empty it in 12 hours. If all pipes are opened together, then the tank will be filled in?  Answers:
A) 60/17 hr
B) 13/12 hr
C) 42/13 hr
D) 13/42hr
E) 14/11 hr
Net part filled in 1 hour = 1/4 + 1/7 - 1/12 = 13/42   The tank will be full in 42/13 hr   Answer is 42/13 hr
<result>C</result>
Pipes A and B can fill a tank in 4 and 7 hours. Pipe C can empty it in 12 hours. If all pipes are opened together, then the tank will be filled in?