<PRE><B>Question 801266</B>
an empty tank can be filled with water in 20 mins using Pipe A or in 30 mins by Pipe B and the tank filled with water can be emptied of water in 40 mins by using Pipe C. When the three pipes A,B and c work together, approximately how long (in minutes) does it take to fill the empty tank with water?

let us consider tank filling or emptying as one job

Pipe A does 1/20 of the job in 1 minute

Pipe B does 1/30 of the job in 1 minute

Pipe C does 1/40 of the job in 1 minute

Pipe C is emptying

1/20 +1/30 -1/40 is the job done in 1 minute

7/120 of the job is done in 1 minute

120/7 minutes is taken to do 1 job

17.14 minutes</PRE>