Two pipes A and B can fill a tank in 15 min and 21 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
A) 14 min 19 sec
B) 14 min 42 sec
C) 14 min 20 sec
D) 14 min 40 sec
E) 15 min 12 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 = 1/21   1/21 : 8/15 :: 1 ; x   x = 8/15 * 1 * 21 = 11 1/5 min = 11 min 12 sec.   The tank will be full in (4 min. + 11 min. 12 sec) = 15 min 12 sec.Answer: 15 min 12 sec
<result>E</result>
Two pipes A and B can fill a tank in 15 min and 21 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?