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

12 minutes 40 sec.

B

11 minutes 35 sec.

C

14 minutes 40 sec.

D

13 minutes 35 sec.