Two pipes A and B can separately fill a tank in 12 and 20 minutes respectively. A third pipe C can drain off 45 liters of water per minute. If all the pipes are opened, the tank can be filled in 15 minutes. What is the capacity of the tank? Choose the correct choice from the following answers:
A) 542	B) 540	C) 675	D) 829	E) 279

Rationale: 1 / 12 + 1 / 20 - 1 / x = 1 / 15 x = 15 15 * 45 = 675 answer : c
Answer: C