Two pipes A and B can separately fill a tank in 12 and 15 minutes respectively. A third pipe C can drain off 42 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?  Answers
A) 542
B) 504
C) 548
D) 829
E) 279
1/12 + 1/15 - 1/x = 1/15   x = 12   12 * 42= 
<gadget id="calculator">12 * 42</gadget>
<output>504</output>
504   Answer: 504
<result>B</result>
Two pipes A and B can separately fill a tank in 12 and 15 minutes respectively. A third pipe C can drain off 42 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?