Pipes A and B can fill a tank in 6 and 4 hours. Pipe C can empty it in 12 hours. If all pipes are opened together, then the tank will be filled in? Choose the correct choice from the following:
A) 3 hr
B) 2 hr
C) 1 hr
D) 9 / 7 hr
E) 5 / 3 hr

Rationale: net part filled in 1 hour = 1 / 6 + 1 / 4 - 1 / 12 = 1 / 3 the tank will be full in 3 hr answer is a
Answer: A