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?

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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 mt 30 sec
(b) 14 mt 40 sec
(c) 10 mt 20 sec
(d) 11 mt 45 sec

Answer: 14 mt 40 sec