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. 17 1/7 mins

  2. 20 mins

  3. 8 mins

  4. none of these

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

Net rate per minute = 1/20 + 1/30 - 1/40 = (6 + 4 - 3)/120 = 7/120. The time taken to fill the tank is the reciprocal: 120/7 = 17 1/7 minutes.