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?
Choose the correct choice from the following options:
A) 10 min 20 sec  B) 11 min 45 sec  C) 12 min 30 sec  D) 14 min 40 sec  E) 15 min 40 sec
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.   ANSWER:14 min 40 sec
<result>D</result>
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?