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.

• Take LCM of 15 and 20 and get the total capacity of tank as 60 .
  Then A will fill 4li in 1 min and B will fill 3 li in 1 min so total will be 7 . A left after 4 min so they worked together for 4 min so 7*4= 28 . now subtract it from 60.(60-38=32). This will be filled by b so time taken would be..10 min 20 sec.
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