Multiple choice general knowledge math & puzzles

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

  1. 20 min

  2. 17 1/7 min

  3. 14 min

  4. none of these

Reveal answer Fill a bubble to check yourself
B 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. This is a standard work rate problem with one outlet.