Multiple choice general knowledge math & puzzles

Pipe A can fill a tank in 20 minutes and Pipe B in 30 mins and Pipe C can empty the same in 40 mins. If all of them work together, find the time taken to fill the tank.

  1. 17 1/7 minutes

  2. 20 minutes

  3. 8 minutes

  4. None of these

Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Combined rate = 1/20 + 1/30 - 1/40 = (6+4-3)/120 = 7/120 tank per minute. Time = 120/7 = 17 1/7 minutes. The claimed answer A is correct.