Two pipes P and Q can fill a cistern in 20 and 15 minutes respectively. Both are opened together, but at the end of 6 minutes the first is turned off. How much longer will the cistern take to fill?
Select the correct option:
A) 11 1/7
B) 10 1/2
C) 11 1/3
D) 11 1/2
E) 11 1/1
5/20+ x/15 = 1   x= 10 1/2.Answer: 10 1/2
<result>B</result>
Two pipes P and Q can fill a cistern in 20 and 15 minutes respectively. Both are opened together, but at the end of 6 minutes the first is turned off. How much longer will the cistern take to fill?