Q. Two pipes A and B can fill a tank in 15 minutes and 20 minutes respectively. Both the pipes are opened together but after 4 minutes, pipe A is turned off. What is the total time required to fill the tank?

• (A) 10 min. 20 sec.
• (B) 11 min. 45 sec.
• (C) 12 min. 30 sec.
• (D) 14 min. 40 sec.