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. 40 min

B. 35 min

C. 45 min

D. 30 min

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