Two pipes P and Q can fill a cistern in 12 and 16 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 the correct choice from the following options.
A) 11 1/2 min
B) 11 1/4 min
C) 11 2/4 min
D) 12 min
E) 11 7/4 min
3/12 + x/16 = 1   x= 12   Answer: 12 min
<result>D</result>
Two pipes P and Q can fill a cistern in 12 and 16 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?