Multiple choice

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

Combined rate: 1/20 + 1/30 - 1/40 = 6/120 + 4/120 - 3/120 = 7/120 tank/min. Time = 120/7 = 17 1/7 minutes. The negative sign for Pipe C is correct because it empties the tank.