Two pipes P and Q can fill a cistern in 16 and 18minutes 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 from the following answers:	A) 11 1/2	B) 13 1/2	C) 13 1/3	D) 13 1/5	E) 13
4/16 + x/18 = 1   x= 13 1/2.Answer: 13 1/2
<result>B</result>
Two pipes P and Q can fill a cistern in 16 and 18minutes 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?