Task: Question Answering
Input (English): Problem: three pipes, a, b, & c are attached to a tank. a & b can fill it in 20 & 30 minutes respectively while c can empty it in 20 minutes. if a, b & c are kept open successively for 1 minute each, how soon will the tank be filled ?
Options: a. 2 hours, b. 4 hours, c. 1 hour 30 min, d. 5 hours, e. 6 hours
Output (English): c