Two pipes A and B can fill a tank in 15 min and 18 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 most appropriate option:
A) 14 min 19 sec B) 13 min 36 sec C) 14 min 20 sec D) 14 min 40 sec E) 14 min 10 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 = 1/18   1/18 : 8/15 :: 1 ; x   x = 8/15 * 1 * 18 = 9 3/5 min = 9 min 36 sec.   The tank will be full in (4 min. + 9 min. 36 sec) = 13 min 36 sec.Answer: 13 min 36 sec
<result>B</result>
Two pipes A and B can fill a tank in 15 min and 18 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?