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?

A. 10 min 20 sec
B. 11 min 45 sec
C. 12 min 30 sec
D. 14 min 40 sec
Explanation:
Part filled in 4 minutes = 4(1/15 + 1/20) = 7/15
Remaining part = 1 – 7/15 = 8/15
Part filled by B in 1 minute = 1/20
1/20 : 8/15 :: 1 ; x
x = 8/15 * 1 * 20 = 10 2/3 min = 10 min 40 sec.
The tank will be full in (4 min. + 10 min. 40 sec) = 14 min 40 sec.