Two pipes P and Q can fill a cistern in 12 and 13 minutes respectively. Both are opened together, but at the end of 3 minutes the first is turned off. How much longer will the cistern take to fill? Choose one:
A) 1/8 B) 1/4 C) 2/4 D) 1/4 E) 3/4
3/12 + x/13   = 1   x= 7 3/4   Answer: 3/4
<result>E</result>
Two pipes P and Q can fill a cistern in 12 and 13 minutes respectively. Both are opened together, but at the end of 3 minutes the first is turned off. How much longer will the cistern take to fill?