Multiple choice general knowledge math & puzzles

Pipe A can fill 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. 20 mins

  2. 8 mins

  3. 17 1/7 mins

  4. 171/6 mins

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

Combined rate = 1/20 + 1/30 - 1/40 = 6/120 + 4/120 - 3/120 = 7/120 tank per minute. Time = 120/7 = 17 1/7 minutes. Pipe C empties, so its contribution is negative. The net rate determines how quickly the tank fills.