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 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 ?
Options: a. 399, b. 540, c. 772, d. 272, e. 121
Output (English): b