Two pipes A and B can fill a cistern in 12 and 15 minutes respectively. Both are opened together but after 6 minutes A is turned off. After how much more time will the cistern be filled?
Choose the correct answer:
A) 8 1/7 B) 8 1/1 C) 8 1/4 D) 1 1/2 E) 8 3/7
6/12 + (6 + x)/15 = 1   x = 1 1/2   Answer: 1 1/2
<result>D</result>
Two pipes A and B can fill a cistern in 12 and 15 minutes respectively. Both are opened together but after 6 minutes A is turned off. After how much more time will the cistern be filled?