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Two pipes P and Q can fill a cistern in 12 min. and 15 min. respectively but a third pipe ‘R’ can empty the full tank in 6 min.  P and Q are kept open for 5 min. in the beginning and then ‘R’ is also opened.  In what time is the cistern emptied ?

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Two pipes P and Q can fill a cistern in 12 min. and 15 min. respectively but a third pipe ‘R’ can empty the full tank in 6 min.  P and Q are kept open for 5 min. in the beginning and then ‘R’ is also opened.  In what time is the cistern emptied ?

A. 30 min
B. 35 min
C. 40 min
D. 45 min