Three pipes, A, B, & C are attached to a tank. A & B can fill it in 20 & 30 minutes respectively while C can empty it in 15 minutes. If A, B & C are kept open successively for 4 minute each, how soon will the tank be filled?	Choose the correct choice:	A) 2 hours	B) 12 hours	C) 3 hours	D) 5 hours	E) 6 hours

Rationale: in three minute 1 / 20 + 1 / 30 - 1 / 15 = 1 / 60 part is filled 12 min - - - - - - - - 1 / 60 parts x min - - - - - - - - - 1 part ( full ) x = 720 min = 12 hours answer : b
Answer: B