Task: Question Answering
Input (English): Problem: 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 ?
Options: a. 549, b. 540, c. 542, d. 672, e. 279
Output (English): d