two pipes a and b can fill a tank in 15 min and 20 min respectively. both the pipes are opened together but after 4 min, pipe a is turned off. what is the total time required to fill the tank?

12 min 30 sec

10 min 20 sec

14 min 40 sec

11 min 45 sec

two pipes a and b can fill a tank in 15 min and 20 min respectively. both the pipes are opened together but after 4 min, pipe a is turned off. what is the total time required to fill the tank?

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