Task: Question Answering
Input (English): Question: Two pipes A and B can separately fill a cistern in 10 and 15 minutes respectively. A person opens both the pipes together when the cistern should have been was full he finds the waste pipe open. He then closes the waste pipe and in another 4 minutes the cistern was full. In what time can the waste pipe empty the cistern when fill?

 Option A: 7 min

 Option B: 8 min

 Option C: 9 min

 Option D: 10 min

 Option E: 12 min
Output (English): Option B