Two pipes P and Q can fill a tank in 15 min and 20 min respectively. Both the pipes are opened together but after 4 min, pipe P is turned off. What is the total time required to fill the tank?  Choices
A) 10 min  B) 12 min  C) 14 min 40 sec  D) 15 min 30 sec  E) 18 min
filled in 4 min = 4(1/15 + 1/20) = 7/15   part = 1-7/15==&gt;8/15   filled by Q in 1 min = 1/20   x=8/15*1*20==&gt; 10 2/3 min==&gt;10 min 40 sec   so tank will be full in (4 min +10 min 40 sec) = 14min 40 sec   ANSWER 14 min 40 sec
<result>C</result>
Two pipes P and Q can fill a tank in 15 min and 20 min respectively. Both the pipes are opened together but after 4 min, pipe P is turned off. What is the total time required to fill the tank?