Two pipes P and Q can fill a cistern in 12 and 15 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?

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Two pipes P and Q can fill a cistern in 12 and 15 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?

A. 9 1/4 min
B. 11 1/4 min
C. 7 1/4 min
D. 8 1/2 min
Explanation:
3/12 + x/15 = 1
x= 11 1/4