Q:

Pipes P and Q can fill a tank in 18 and 27 minutes, respectively, whereas Pipe R can empty the full tank in 54 minutes.  P and Q were opened together for 6 minutes and then   closed and R was opened. The tank was emptied by R alone in :

• 1
  35 minutes
• 2
  45 minutes
• 3
  30 minutes
• 4
  40 minutes