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Two pipes A and B can fill a cistern in 12 and 15 minutes respectively. Both are opened together but after 3 minutes A is turned off. After how much more time will the cistern be filled?

Two pipes A and B can fill a cistern in 12 and 15 minutes respectively. Both are opened together but after 3 minutes A is turned off. After how much more time will the cistern be filled?

A. 3 1/4 min
B. 5 1/4 min
C. 8 1/4 min
D. 9 1/4 min
Explanation:
3/12 + (3 + x)/15 = 1
x = 8 1/4