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

  1. 20 1/7 mins

  2. 8 mins

  3. 17 1/7 mins

  4. none

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

Combined rate = 1/20 + 1/30 - 1/40 = (6 + 4 - 3) / 120 = 7/120. The time required is the reciprocal of the rate: 120 / 7 = 17 1/7 minutes.