Two pipes A and B can separately fill a tank in 12 and 15 minutes respectively. A third pipe C can drain off 56 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 one.
A) 549
B) 540
C) 542
D) 672
E) 279
1/12 + 1/15 - 1/x = 1/15   x = 12   12 * 56= 
<gadget id="calculator">12 * 56</gadget>
<output>672</output>
672   Answer: 672
<result>D</result>
Two pipes A and B can separately fill a tank in 12 and 15 minutes respectively. A third pipe C can drain off 56 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?