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