Multiple choice

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. 30 minutes

  2. 40 minutes

  3. 35 minutes

  4. 45 minutes
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

P fills 1/18 per minute, Q fills 1/27 per minute. Together in 6 minutes: 6*(1/18+1/27) = 6*(5/54) = 30/54 = 5/9 tank filled. Remaining tank to empty = 5/9. R empties 1/54 per minute. Time = (5/9)/(1/54) = (5/9)*54 = 30 minutes.

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