Two pipes P and Q can fill a cistern in 12 and 15 minutes respectively. Both are opened together, but at the end of 4 minutes the first is turned off. How much longer will the cistern take to fill?  Choose the correct choice.
A) 10 1/7	B) 10 1/4	C) 10 1/3	D) 10 1/2	E) 10
4/12 + x/15 = 1   x= 10.Answer: 10
<result>E</result>
Two pipes P and Q can fill a cistern in 12 and 15 minutes respectively. Both are opened together, but at the end of 4 minutes the first is turned off. How much longer will the cistern take to fill?