Two pipes P and Q can fill a cistern in 15 and 10 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?
Choose the correct option:  A) 6 1/7  B) 6 1/4  C) 6  D) 6 1/2  E) 6 1/1
6/15+ x/10= 1   x= 6.Answer: 6
<result>C</result>
Two pipes P and Q can fill a cistern in 15 and 10 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?