Two pipes A and B can separately fill a tank in 10 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 correct choice:
A) 6min  B) 9min  C) 7min  D) 8min  E) 10min
4/10 + x/15 = 1   x = 9   ANSWER:9min
<result>B</result>
Two pipes A and B can separately fill a tank in 10 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?