Two pipes A and B can separately fill a tank 12 minutes and 15 minutes respectively. Both the pipes are opened together but 4 minutes after the start the pipe A is turned off. How much time will it take to fill the tank?  Choose the most appropriate option:  A) 5min  B) 6min  C) 10min  D) 8min  E) 9min
4/12 + x/15 = 1   x = 10   ANSWER:10min
<result>C</result>
Two pipes A and B can separately fill a tank 12 minutes and 15 minutes respectively. Both the pipes are opened together but 4 minutes after the start the pipe A is turned off. How much time will it take to fill the tank?