Two fill pipes A and B can fill a cistern in 12 and 16 minutes respectively. Both fill pipes are opened together, but 4 minutes before the cistern is full, one pipe A is closed. How much time will the cistern take to fill ? Choose the correct choice from the following options.
A) 9 1⁄7min.	B) 3 1⁄3min.	C) 5 min.	D) 3 min.	E) None of these
Let cistern will be full in x min. Then,   part filled by B in x min + part filled by A in (x – 4)min = 1   ⇒ x/16+x−4/12=1   ⇒ x = 64⁄7 = 91⁄7 hours.   9 1⁄7min.nswer 9 1⁄7min.
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Two fill pipes A and B can fill a cistern in 12 and 16 minutes respectively. Both fill pipes are opened together, but 4 minutes before the cistern is full, one pipe A is closed. How much time will the cistern take to fill ?