Two pipes A and B can fill a cistern 20 and 24 minutes respectively. Both pipes being opened, find when the first pipe must be turned off, so that the cistern may be filled in 12 minutes?
Part of the cistern filled by pipe B in 12 minutes = 12/24 = 1/2;
That means pipe A should fill ½ of the cistern before it is turned off.
Time required for pipe A to fill half of the cistern = 10 minutes
Answer : After 10 minutes.